diff --git a/src/lib/__tests__/notificationSound.test.ts b/src/lib/__tests__/notificationSound.test.ts
new file mode 100644
index 0000000..a34b0a6
--- /dev/null
+++ b/src/lib/__tests__/notificationSound.test.ts
@@ -0,0 +1,155 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+
+// Mock Web Audio API nodes
+function createMockOscillator() {
+  return {
+    type: 'sine' as OscillatorType,
+    frequency: { value: 0 },
+    connect: vi.fn(),
+    start: vi.fn(),
+    stop: vi.fn(),
+  };
+}
+
+function createMockGain() {
+  return {
+    gain: {
+      setValueAtTime: vi.fn(),
+      linearRampToValueAtTime: vi.fn(),
+      exponentialRampToValueAtTime: vi.fn(),
+    },
+    connect: vi.fn(),
+  };
+}
+
+describe('playNotificationSound', () => {
+  let mockCtx: Record<string, unknown>;
+  let oscillators: ReturnType<typeof createMockOscillator>[];
+  let gains: ReturnType<typeof createMockGain>[];
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  let origAudioContext: any;
+
+  beforeEach(() => {
+    vi.resetModules();
+
+    oscillators = [];
+    gains = [];
+
+    mockCtx = {
+      currentTime: 0,
+      state: 'running',
+      destination: {},
+      resume: vi.fn(),
+      createOscillator: vi.fn(() => {
+        const osc = createMockOscillator();
+        oscillators.push(osc);
+        return osc;
+      }),
+      createGain: vi.fn(() => {
+        const gain = createMockGain();
+        gains.push(gain);
+        return gain;
+      }),
+    };
+
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    origAudioContext = (globalThis as any).AudioContext;
+    // Use a proper class so `new AudioContext()` works
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (globalThis as any).AudioContext = class {
+      constructor() {
+        return mockCtx;
+      }
+    };
+  });
+
+  afterEach(() => {
+    if (origAudioContext === undefined) {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      delete (globalThis as any).AudioContext;
+    } else {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (globalThis as any).AudioContext = origAudioContext;
+    }
+  });
+
+  async function getPlayFn() {
+    const mod = await import('../notificationSound');
+    return mod.playNotificationSound;
+  }
+
+  it('creates two oscillators for the two-tone chime', async () => {
+    const play = await getPlayFn();
+    play();
+    expect(oscillators).toHaveLength(2);
+    expect(gains).toHaveLength(2);
+  });
+
+  it('uses correct frequencies (C5 and E5)', async () => {
+    const play = await getPlayFn();
+    play();
+    expect(oscillators[0].frequency.value).toBe(523.25);
+    expect(oscillators[1].frequency.value).toBe(659.25);
+  });
+
+  it('sets oscillator type to sine', async () => {
+    const play = await getPlayFn();
+    play();
+    oscillators.forEach((osc) => {
+      expect(osc.type).toBe('sine');
+    });
+  });
+
+  it('connects oscillators through gain nodes to destination', async () => {
+    const play = await getPlayFn();
+    play();
+    oscillators.forEach((osc, i) => {
+      expect(osc.connect).toHaveBeenCalledWith(gains[i]);
+      expect(gains[i].connect).toHaveBeenCalledWith(mockCtx.destination);
+    });
+  });
+
+  it('starts and stops each oscillator', async () => {
+    const play = await getPlayFn();
+    play();
+    oscillators.forEach((osc) => {
+      expect(osc.start).toHaveBeenCalledTimes(1);
+      expect(osc.stop).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  it('applies gain envelope with the given volume', async () => {
+    const play = await getPlayFn();
+    play(0.5);
+    expect(gains[0].gain.linearRampToValueAtTime).toHaveBeenCalledWith(
+      0.5,
+      expect.any(Number),
+    );
+  });
+
+  it('uses default volume of 0.3', async () => {
+    const play = await getPlayFn();
+    play();
+    expect(gains[0].gain.linearRampToValueAtTime).toHaveBeenCalledWith(
+      0.3,
+      expect.any(Number),
+    );
+  });
+
+  it('resumes suspended AudioContext', async () => {
+    mockCtx.state = 'suspended';
+    const play = await getPlayFn();
+    play();
+    expect(mockCtx.resume).toHaveBeenCalled();
+  });
+
+  it('does nothing when AudioContext is unavailable', async () => {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    delete (globalThis as any).AudioContext;
+    const play = await getPlayFn();
+    expect(() => play()).not.toThrow();
+  });
+});