Audit Bluetooth audio routing during recording (headset mic, media playback)

[Pivii]

Context

Android currently uses default AudioManager routing with no explicit Bluetooth handling. On iOS this caused real bugs: HFP (.allowBluetooth) hijacked AirPods play/pause and degraded quality; the fix was A2DP-only output + built-in mic (v1.6.0-beta.1 #85), plus an earpiece-routing fix (#96).

The Android equivalent question: when a BT headset is connected, does AudioRecord with MediaRecorder.AudioSource.MIC use the (low-quality SCO) headset mic or the built-in mic? Does music keep playing / duck properly?

What to do

1. Test matrix: BT earbuds + music playing, BT earbuds idle, wired headset, no headset — on Pixel 4
2. Decide policy (recommendation: built-in mic for quality, like iOS; don't start SCO)
3. Implement explicit routing if defaults are wrong (setPreferredDevice, avoid startBluetoothSco)
4. Verify media keys / playback aren't hijacked during recording

Acceptance criteria

• Documented behavior per scenario (comment on this issue)
• Recording quality is consistent with and without BT devices connected
• Music resumes/ducks correctly around a dictation

[Pivii]

#25 audit milestone — deterministic built-in-input policy in PR #52

Current code audit confirmed that AudioCaptureManager used AudioSource.MIC with default/OEM input routing and no explicit route verification. The repository contains no startBluetoothSco, communication-device, output-routing, or media-control call.

PR #52 now requests TYPE_BUILTIN_MIC on each initialized AudioRecord before capture starts, while deliberately leaving output/media routing to Android. It falls back if no built-in mic exists or Android rejects the preference.

Evidence on exact head e1027bf:

• clean lint/JVM/APK gate passed; 277 tests, no failures/errors/skips
• selector sabotage to Bluetooth SCO failed all 3 focused tests
• fresh-context review approved after privacy-safe logging correction
• Pixel 4 Android 15 / ARM64 real capture reported preference accepted and actual routedType=15 (TYPE_BUILTIN_MIC)
• no Dictus fatal exception/ANR; no audio/transcript retained

Physical inventory finding: the attached Pixel currently has zero bonded Bluetooth devices and no wired headset was available. Therefore the parent issue remains open for its accessory matrix: BT idle, BT + media playing, wired, acoustic consistency, duck/resume, and media-key behavior. Those scenarios are not claimed by #52.