diff --git a/apps/api/src/speech/dto/index.ts b/apps/api/src/speech/dto/index.ts
new file mode 100644
index 0000000..8b644f8
--- /dev/null
+++ b/apps/api/src/speech/dto/index.ts
@@ -0,0 +1,8 @@
+/**
+ * Speech DTOs barrel export
+ *
+ * Issue #398
+ */
+
+export { TranscribeDto } from "./transcribe.dto";
+export { SynthesizeDto } from "./synthesize.dto";
diff --git a/apps/api/src/speech/dto/synthesize.dto.ts b/apps/api/src/speech/dto/synthesize.dto.ts
new file mode 100644
index 0000000..171dc0e
--- /dev/null
+++ b/apps/api/src/speech/dto/synthesize.dto.ts
@@ -0,0 +1,85 @@
+/**
+ * SynthesizeDto
+ *
+ * DTO for text-to-speech synthesis requests.
+ * The text field is validated by TextValidationPipe for length/emptiness.
+ * Additional options control voice, speed, format, and tier selection.
+ *
+ * Issue #398
+ */
+
+import { IsString, IsOptional, IsNumber, IsIn, Min, Max, MaxLength } from "class-validator";
+import { Type } from "class-transformer";
+import type { AudioFormat, SpeechTier } from "../interfaces/speech-types";
+
+/**
+ * Valid audio output formats for TTS synthesis.
+ */
+const VALID_AUDIO_FORMATS: readonly AudioFormat[] = [
+  "mp3",
+  "wav",
+  "opus",
+  "flac",
+  "aac",
+  "pcm",
+] as const;
+
+/**
+ * Valid TTS tiers for provider selection.
+ */
+const VALID_SPEECH_TIERS: readonly SpeechTier[] = ["default", "premium", "fallback"] as const;
+
+export class SynthesizeDto {
+  /**
+   * Text to convert to speech.
+   * Validated separately by TextValidationPipe for length and emptiness.
+   */
+  @IsString({ message: "text must be a string" })
+  @MaxLength(4096, { message: "text must not exceed 4096 characters" })
+  text!: string;
+
+  /**
+   * Voice ID to use for synthesis.
+   * Available voices depend on the selected tier and provider.
+   * If omitted, the default voice from speech config is used.
+   */
+  @IsOptional()
+  @IsString({ message: "voice must be a string" })
+  @MaxLength(100, { message: "voice must not exceed 100 characters" })
+  voice?: string;
+
+  /**
+   * Speech speed multiplier (0.5 to 2.0).
+   * 1.0 is normal speed, <1.0 is slower, >1.0 is faster.
+   */
+  @IsOptional()
+  @Type(() => Number)
+  @IsNumber({}, { message: "speed must be a number" })
+  @Min(0.5, { message: "speed must be at least 0.5" })
+  @Max(2.0, { message: "speed must not exceed 2.0" })
+  speed?: number;
+
+  /**
+   * Desired audio output format.
+   * Supported: mp3, wav, opus, flac, aac, pcm.
+   * If omitted, the default format from speech config is used.
+   */
+  @IsOptional()
+  @IsString({ message: "format must be a string" })
+  @IsIn(VALID_AUDIO_FORMATS, {
+    message: `format must be one of: ${VALID_AUDIO_FORMATS.join(", ")}`,
+  })
+  format?: AudioFormat;
+
+  /**
+   * TTS tier to use for synthesis.
+   * Controls which provider is used: default (Kokoro), premium (Chatterbox), or fallback (Piper).
+   * If the selected tier is unavailable, the service falls back to the next available tier.
+   */
+  @IsOptional()
+  @IsString({ message: "tier must be a string" })
+  @IsIn(VALID_SPEECH_TIERS, {
+    message: `tier must be one of: ${VALID_SPEECH_TIERS.join(", ")}`,
+  })
+  tier?: SpeechTier;
+}
diff --git a/apps/api/src/speech/dto/transcribe.dto.ts b/apps/api/src/speech/dto/transcribe.dto.ts
new file mode 100644
index 0000000..8a7bbe4
--- /dev/null
+++ b/apps/api/src/speech/dto/transcribe.dto.ts
@@ -0,0 +1,54 @@
+/**
+ * TranscribeDto
+ *
+ * DTO for speech-to-text transcription requests.
+ * Supports optional language and model overrides.
+ *
+ * The audio file itself is handled by Multer (FileInterceptor)
+ * and validated by AudioValidationPipe.
+ *
+ * Issue #398
+ */
+
+import { IsString, IsOptional, IsNumber, Min, Max, MaxLength } from "class-validator";
+import { Type } from "class-transformer";
+
+export class TranscribeDto {
+  /**
+   * Language code for transcription (e.g., "en", "fr", "de").
+   * If omitted, the default from speech config is used.
+   */
+  @IsOptional()
+  @IsString({ message: "language must be a string" })
+  @MaxLength(10, { message: "language must not exceed 10 characters" })
+  language?: string;
+
+  /**
+   * Model override for transcription.
+   * If omitted, the default model from speech config is used.
+   */
+  @IsOptional()
+  @IsString({ message: "model must be a string" })
+  @MaxLength(200, { message: "model must not exceed 200 characters" })
+  model?: string;
+
+  /**
+   * Optional prompt to guide the transcription model.
+   * Useful for providing context or expected vocabulary.
+   */
+  @IsOptional()
+  @IsString({ message: "prompt must be a string" })
+  @MaxLength(1000, { message: "prompt must not exceed 1000 characters" })
+  prompt?: string;
+
+  /**
+   * Temperature for transcription (0.0 to 1.0).
+   * Lower values produce more deterministic results.
+   */
+  @IsOptional()
+  @Type(() => Number)
+  @IsNumber({}, { message: "temperature must be a number" })
+  @Min(0, { message: "temperature must be at least 0" })
+  @Max(1, { message: "temperature must not exceed 1" })
+  temperature?: number;
+}
diff --git a/apps/api/src/speech/pipes/audio-validation.pipe.spec.ts b/apps/api/src/speech/pipes/audio-validation.pipe.spec.ts
new file mode 100644
index 0000000..fc9c5ab
--- /dev/null
+++ b/apps/api/src/speech/pipes/audio-validation.pipe.spec.ts
@@ -0,0 +1,205 @@
+/**
+ * AudioValidationPipe Tests
+ *
+ * Issue #398: Validates uploaded audio files for MIME type and file size.
+ * Tests cover valid types, invalid types, size limits, and edge cases.
+ */
+
+import { describe, it, expect, beforeEach } from "vitest";
+import { BadRequestException } from "@nestjs/common";
+import { AudioValidationPipe } from "./audio-validation.pipe";
+
+/**
+ * Helper to create a mock Express.Multer.File object.
+ */
+function createMockFile(overrides: Partial<Express.Multer.File> = {}): Express.Multer.File {
+  return {
+    fieldname: "file",
+    originalname: "test.mp3",
+    encoding: "7bit",
+    mimetype: "audio/mpeg",
+    size: 1024,
+    destination: "",
+    filename: "",
+    path: "",
+    buffer: Buffer.from("fake-audio-data"),
+    stream: undefined as never,
+    ...overrides,
+  };
+}
+
+describe("AudioValidationPipe", () => {
+  // ==========================================
+  // Default config (25MB max)
+  // ==========================================
+  describe("with default config", () => {
+    let pipe: AudioValidationPipe;
+
+    beforeEach(() => {
+      pipe = new AudioValidationPipe();
+    });
+
+    // ==========================================
+    // MIME type validation
+    // ==========================================
+    describe("MIME type validation", () => {
+      it("should accept audio/wav", () => {
+        const file = createMockFile({ mimetype: "audio/wav" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/mp3", () => {
+        const file = createMockFile({ mimetype: "audio/mp3" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/mpeg", () => {
+        const file = createMockFile({ mimetype: "audio/mpeg" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/webm", () => {
+        const file = createMockFile({ mimetype: "audio/webm" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/ogg", () => {
+        const file = createMockFile({ mimetype: "audio/ogg" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/flac", () => {
+        const file = createMockFile({ mimetype: "audio/flac" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept audio/x-m4a", () => {
+        const file = createMockFile({ mimetype: "audio/x-m4a" });
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should reject unsupported MIME types with descriptive error", () => {
+        const file = createMockFile({ mimetype: "video/mp4" });
+        expect(() => pipe.transform(file)).toThrow(BadRequestException);
+        expect(() => pipe.transform(file)).toThrow(/Unsupported audio format.*video\/mp4/);
+      });
+
+      it("should reject application/octet-stream", () => {
+        const file = createMockFile({ mimetype: "application/octet-stream" });
+        expect(() => pipe.transform(file)).toThrow(BadRequestException);
+      });
+
+      it("should reject text/plain", () => {
+        const file = createMockFile({ mimetype: "text/plain" });
+        expect(() => pipe.transform(file)).toThrow(BadRequestException);
+      });
+
+      it("should reject image/png", () => {
+        const file = createMockFile({ mimetype: "image/png" });
+        expect(() => pipe.transform(file)).toThrow(BadRequestException);
+      });
+
+      it("should include supported formats in error message", () => {
+        const file = createMockFile({ mimetype: "video/mp4" });
+        try {
+          pipe.transform(file);
+          expect.fail("Expected BadRequestException");
+        } catch (error) {
+          expect(error).toBeInstanceOf(BadRequestException);
+          const response = (error as BadRequestException).getResponse();
+          const message =
+            typeof response === "string" ? response : (response as Record<string, unknown>).message;
+          expect(message).toContain("audio/wav");
+          expect(message).toContain("audio/mpeg");
+        }
+      });
+    });
+
+    // ==========================================
+    // File size validation
+    // ==========================================
+    describe("file size validation", () => {
+      it("should accept files under the size limit", () => {
+        const file = createMockFile({ size: 1024 * 1024 }); // 1MB
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should accept files exactly at the size limit", () => {
+        const file = createMockFile({ size: 25_000_000 }); // 25MB (default)
+        expect(pipe.transform(file)).toBe(file);
+      });
+
+      it("should reject files exceeding the size limit", () => {
+        const file = createMockFile({ size: 25_000_001 }); // 1 byte over
+        expect(() => pipe.transform(file)).toThrow(BadRequestException);
+        expect(() => pipe.transform(file)).toThrow(/exceeds maximum/);
+      });
+
+      it("should include human-readable sizes in error message", () => {
+        const file = createMockFile({ size: 30_000_000 }); // 30MB
+        try {
+          pipe.transform(file);
+          expect.fail("Expected BadRequestException");
+        } catch (error) {
+          expect(error).toBeInstanceOf(BadRequestException);
+          const response = (error as BadRequestException).getResponse();
+          const message =
+            typeof response === "string" ? response : (response as Record<string, unknown>).message;
+          // Should show something like "28.6 MB" and "23.8 MB"
+          expect(message).toContain("MB");
+        }
+      });
+
+      it("should accept zero-size files (MIME check still applies)", () => {
+        const file = createMockFile({ size: 0 });
+        expect(pipe.transform(file)).toBe(file);
+      });
+    });
+
+    // ==========================================
+    // Edge cases
+    // ==========================================
+    describe("edge cases", () => {
+      it("should throw if no file is provided (null)", () => {
+        expect(() => pipe.transform(null as unknown as Express.Multer.File)).toThrow(
+          BadRequestException
+        );
+        expect(() => pipe.transform(null as unknown as Express.Multer.File)).toThrow(
+          /No audio file provided/
+        );
+      });
+
+      it("should throw if no file is provided (undefined)", () => {
+        expect(() => pipe.transform(undefined as unknown as Express.Multer.File)).toThrow(
+          BadRequestException
+        );
+      });
+    });
+  });
+
+  // ==========================================
+  // Custom config
+  // ==========================================
+  describe("with custom config", () => {
+    it("should use custom max file size", () => {
+      const pipe = new AudioValidationPipe({ maxFileSize: 1_000_000 }); // 1MB
+      const smallFile = createMockFile({ size: 500_000 });
+      expect(pipe.transform(smallFile)).toBe(smallFile);
+
+      const largeFile = createMockFile({ size: 1_000_001 });
+      expect(() => pipe.transform(largeFile)).toThrow(BadRequestException);
+    });
+
+    it("should allow overriding accepted MIME types", () => {
+      const pipe = new AudioValidationPipe({
+        allowedMimeTypes: ["audio/wav"],
+      });
+
+      const wavFile = createMockFile({ mimetype: "audio/wav" });
+      expect(pipe.transform(wavFile)).toBe(wavFile);
+
+      const mp3File = createMockFile({ mimetype: "audio/mpeg" });
+      expect(() => pipe.transform(mp3File)).toThrow(BadRequestException);
+    });
+  });
+});
diff --git a/apps/api/src/speech/pipes/audio-validation.pipe.ts b/apps/api/src/speech/pipes/audio-validation.pipe.ts
new file mode 100644
index 0000000..f5491d6
--- /dev/null
+++ b/apps/api/src/speech/pipes/audio-validation.pipe.ts
@@ -0,0 +1,102 @@
+/**
+ * AudioValidationPipe
+ *
+ * NestJS PipeTransform that validates uploaded audio files.
+ * Checks MIME type against an allow-list and file size against a configurable maximum.
+ *
+ * Usage:
+ * ```typescript
+ * @Post('transcribe')
+ * @UseInterceptors(FileInterceptor('file'))
+ * async transcribe(
+ *   @UploadedFile(new AudioValidationPipe()) file: Express.Multer.File,
+ * ) { ... }
+ * ```
+ *
+ * Issue #398
+ */
+
+import { BadRequestException } from "@nestjs/common";
+import type { PipeTransform } from "@nestjs/common";
+
+/**
+ * Default accepted MIME types for audio uploads.
+ */
+const DEFAULT_ALLOWED_MIME_TYPES: readonly string[] = [
+  "audio/wav",
+  "audio/mp3",
+  "audio/mpeg",
+  "audio/webm",
+  "audio/ogg",
+  "audio/flac",
+  "audio/x-m4a",
+] as const;
+
+/**
+ * Default maximum upload size in bytes (25 MB).
+ */
+const DEFAULT_MAX_FILE_SIZE = 25_000_000;
+
+/**
+ * Options for customizing AudioValidationPipe behavior.
+ */
+export interface AudioValidationPipeOptions {
+  /** Maximum file size in bytes. Defaults to 25 MB. */
+  maxFileSize?: number;
+
+  /** List of accepted MIME types. Defaults to common audio formats. */
+  allowedMimeTypes?: string[];
+}
+
+/**
+ * Format bytes into a human-readable string (e.g., "25.0 MB").
+ */
+function formatBytes(bytes: number): string {
+  if (bytes < 1024) {
+    return `${String(bytes)} B`;
+  }
+  if (bytes < 1024 * 1024) {
+    return `${(bytes / 1024).toFixed(1)} KB`;
+  }
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+export class AudioValidationPipe implements PipeTransform<Express.Multer.File | undefined> {
+  private readonly maxFileSize: number;
+  private readonly allowedMimeTypes: readonly string[];
+
+  constructor(options?: AudioValidationPipeOptions) {
+    this.maxFileSize = options?.maxFileSize ?? DEFAULT_MAX_FILE_SIZE;
+    this.allowedMimeTypes = options?.allowedMimeTypes ?? DEFAULT_ALLOWED_MIME_TYPES;
+  }
+
+  /**
+   * Validate the uploaded file's MIME type and size.
+   *
+   * @param file - The uploaded file from Multer
+   * @returns The validated file, unchanged
+   * @throws {BadRequestException} If the file is missing, has an unsupported MIME type, or exceeds the size limit
+   */
+  transform(file: Express.Multer.File | undefined): Express.Multer.File {
+    if (!file) {
+      throw new BadRequestException("No audio file provided");
+    }
+
+    // Validate MIME type
+    if (!this.allowedMimeTypes.includes(file.mimetype)) {
+      throw new BadRequestException(
+        `Unsupported audio format: ${file.mimetype}. ` +
+          `Supported formats: ${this.allowedMimeTypes.join(", ")}`
+      );
+    }
+
+    // Validate file size
+    if (file.size > this.maxFileSize) {
+      throw new BadRequestException(
+        `File size ${formatBytes(file.size)} exceeds maximum allowed size of ${formatBytes(this.maxFileSize)}`
+      );
+    }
+
+    return file;
+  }
+}
diff --git a/apps/api/src/speech/pipes/index.ts b/apps/api/src/speech/pipes/index.ts
new file mode 100644
index 0000000..8bb0ab5
--- /dev/null
+++ b/apps/api/src/speech/pipes/index.ts
@@ -0,0 +1,10 @@
+/**
+ * Speech Pipes barrel export
+ *
+ * Issue #398
+ */
+
+export { AudioValidationPipe } from "./audio-validation.pipe";
+export type { AudioValidationPipeOptions } from "./audio-validation.pipe";
+export { TextValidationPipe } from "./text-validation.pipe";
+export type { TextValidationPipeOptions } from "./text-validation.pipe";
diff --git a/apps/api/src/speech/pipes/text-validation.pipe.spec.ts b/apps/api/src/speech/pipes/text-validation.pipe.spec.ts
new file mode 100644
index 0000000..33a263c
--- /dev/null
+++ b/apps/api/src/speech/pipes/text-validation.pipe.spec.ts
@@ -0,0 +1,136 @@
+/**
+ * TextValidationPipe Tests
+ *
+ * Issue #398: Validates text input for TTS synthesis.
+ * Tests cover text length, empty text, whitespace, and configurable limits.
+ */
+
+import { describe, it, expect, beforeEach } from "vitest";
+import { BadRequestException } from "@nestjs/common";
+import { TextValidationPipe } from "./text-validation.pipe";
+
+describe("TextValidationPipe", () => {
+  // ==========================================
+  // Default config (4096 max length)
+  // ==========================================
+  describe("with default config", () => {
+    let pipe: TextValidationPipe;
+
+    beforeEach(() => {
+      pipe = new TextValidationPipe();
+    });
+
+    // ==========================================
+    // Valid text
+    // ==========================================
+    describe("valid text", () => {
+      it("should accept normal text", () => {
+        const text = "Hello, world!";
+        expect(pipe.transform(text)).toBe(text);
+      });
+
+      it("should accept text at exactly the max length", () => {
+        const text = "a".repeat(4096);
+        expect(pipe.transform(text)).toBe(text);
+      });
+
+      it("should accept single character text", () => {
+        expect(pipe.transform("a")).toBe("a");
+      });
+
+      it("should accept text with unicode characters", () => {
+        const text = "Hello, world! 你好世界";
+        expect(pipe.transform(text)).toBe(text);
+      });
+
+      it("should accept multi-line text", () => {
+        const text = "Line one.\nLine two.\nLine three.";
+        expect(pipe.transform(text)).toBe(text);
+      });
+    });
+
+    // ==========================================
+    // Text length validation
+    // ==========================================
+    describe("text length validation", () => {
+      it("should reject text exceeding max length", () => {
+        const text = "a".repeat(4097);
+        expect(() => pipe.transform(text)).toThrow(BadRequestException);
+        expect(() => pipe.transform(text)).toThrow(/exceeds maximum/);
+      });
+
+      it("should include length details in error message", () => {
+        const text = "a".repeat(5000);
+        try {
+          pipe.transform(text);
+          expect.fail("Expected BadRequestException");
+        } catch (error) {
+          expect(error).toBeInstanceOf(BadRequestException);
+          const response = (error as BadRequestException).getResponse();
+          const message =
+            typeof response === "string" ? response : (response as Record<string, unknown>).message;
+          expect(message).toContain("5000");
+          expect(message).toContain("4096");
+        }
+      });
+    });
+
+    // ==========================================
+    // Empty text validation
+    // ==========================================
+    describe("empty text validation", () => {
+      it("should reject empty string", () => {
+        expect(() => pipe.transform("")).toThrow(BadRequestException);
+        expect(() => pipe.transform("")).toThrow(/Text cannot be empty/);
+      });
+
+      it("should reject whitespace-only string", () => {
+        expect(() => pipe.transform("   ")).toThrow(BadRequestException);
+        expect(() => pipe.transform("   ")).toThrow(/Text cannot be empty/);
+      });
+
+      it("should reject tabs and newlines only", () => {
+        expect(() => pipe.transform("\t\n\r")).toThrow(BadRequestException);
+      });
+
+      it("should reject null", () => {
+        expect(() => pipe.transform(null as unknown as string)).toThrow(BadRequestException);
+      });
+
+      it("should reject undefined", () => {
+        expect(() => pipe.transform(undefined as unknown as string)).toThrow(BadRequestException);
+      });
+    });
+
+    // ==========================================
+    // Text with leading/trailing whitespace
+    // ==========================================
+    describe("whitespace handling", () => {
+      it("should accept text with leading/trailing whitespace (preserves it)", () => {
+        const text = "  Hello, world!  ";
+        expect(pipe.transform(text)).toBe(text);
+      });
+    });
+  });
+
+  // ==========================================
+  // Custom config
+  // ==========================================
+  describe("with custom config", () => {
+    it("should use custom max text length", () => {
+      const pipe = new TextValidationPipe({ maxTextLength: 100 });
+
+      const shortText = "Hello";
+      expect(pipe.transform(shortText)).toBe(shortText);
+
+      const longText = "a".repeat(101);
+      expect(() => pipe.transform(longText)).toThrow(BadRequestException);
+    });
+
+    it("should accept text at exact custom limit", () => {
+      const pipe = new TextValidationPipe({ maxTextLength: 50 });
+      const text = "a".repeat(50);
+      expect(pipe.transform(text)).toBe(text);
+    });
+  });
+});
diff --git a/apps/api/src/speech/pipes/text-validation.pipe.ts b/apps/api/src/speech/pipes/text-validation.pipe.ts
new file mode 100644
index 0000000..36796d1
--- /dev/null
+++ b/apps/api/src/speech/pipes/text-validation.pipe.ts
@@ -0,0 +1,65 @@
+/**
+ * TextValidationPipe
+ *
+ * NestJS PipeTransform that validates text input for TTS synthesis.
+ * Checks that text is non-empty and within the configurable maximum length.
+ *
+ * Usage:
+ * ```typescript
+ * @Post('synthesize')
+ * async synthesize(
+ *   @Body('text', new TextValidationPipe()) text: string,
+ * ) { ... }
+ * ```
+ *
+ * Issue #398
+ */
+
+import { BadRequestException } from "@nestjs/common";
+import type { PipeTransform } from "@nestjs/common";
+
+/**
+ * Default maximum text length for TTS input (4096 characters).
+ */
+const DEFAULT_MAX_TEXT_LENGTH = 4096;
+
+/**
+ * Options for customizing TextValidationPipe behavior.
+ */
+export interface TextValidationPipeOptions {
+  /** Maximum text length in characters. Defaults to 4096. */
+  maxTextLength?: number;
+}
+
+export class TextValidationPipe implements PipeTransform<string | null | undefined> {
+  private readonly maxTextLength: number;
+
+  constructor(options?: TextValidationPipeOptions) {
+    this.maxTextLength = options?.maxTextLength ?? DEFAULT_MAX_TEXT_LENGTH;
+  }
+
+  /**
+   * Validate the text input for TTS synthesis.
+   *
+   * @param text - The text to validate
+   * @returns The validated text, unchanged
+   * @throws {BadRequestException} If text is empty, whitespace-only, or exceeds the max length
+   */
+  transform(text: string | null | undefined): string {
+    if (text === null || text === undefined) {
+      throw new BadRequestException("Text cannot be empty");
+    }
+
+    if (text.trim().length === 0) {
+      throw new BadRequestException("Text cannot be empty");
+    }
+
+    if (text.length > this.maxTextLength) {
+      throw new BadRequestException(
+        `Text length ${String(text.length)} exceeds maximum allowed length of ${String(this.maxTextLength)} characters`
+      );
+    }
+
+    return text;
+  }
+}