fix(audio): defer MODE_IN_COMMUNICATION to call start, restore on end

Root cause: MainActivity set MODE_IN_COMMUNICATION at app launch,
hijacking system audio routing immediately — BT A2DP music dropped to
earpiece, and the pre-existing communication mode confused subsequent
setCommunicationDevice calls for BT SCO.

Fix: MainActivity now only sets volumes. MODE_IN_COMMUNICATION is set
via JNI right before Oboe audio_start() in CallEngine, and MODE_NORMAL
is restored after audio_stop() when the call ends.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/gen/android/app/src/main/java/com/wzp/desktop/MainActivity.kt

@@ -72,18 +72,22 @@ class MainActivity : TauriActivity() {
    * STREAM_VOICE_CALL volume is cranked to max since the in-call volume
    * slider is separate from media volume on most devices.
    */
+  /**
+   * Pre-flight: only set volumes. Do NOT set MODE_IN_COMMUNICATION here —
+   * that hijacks the entire audio routing (music stops, BT A2DP drops to
+   * earpiece) even before a call starts. The Rust side sets the mode via
+   * JNI when the call engine actually starts, and restores MODE_NORMAL
+   * when the call ends.
+   */
   private fun configureAudioForCall() {
     try {
       val am = getSystemService(Context.AUDIO_SERVICE) as AudioManager
-      Log.i(TAG, "audio state before: mode=${am.mode} speaker=${am.isSpeakerphoneOn} " +
+      Log.i(TAG, "audio state: mode=${am.mode} speaker=${am.isSpeakerphoneOn} " +
         "voiceVol=${am.getStreamVolume(AudioManager.STREAM_VOICE_CALL)}/" +
         "${am.getStreamMaxVolume(AudioManager.STREAM_VOICE_CALL)} " +
         "musicVol=${am.getStreamVolume(AudioManager.STREAM_MUSIC)}/" +
         "${am.getStreamMaxVolume(AudioManager.STREAM_MUSIC)}")
 
-      am.mode = AudioManager.MODE_IN_COMMUNICATION
-      am.isSpeakerphoneOn = false   // default: handset / earpiece
-
       // Crank both voice-call and music volumes so nothing silent slips
       // through regardless of which stream actually ends up driving.
       val maxVoice = am.getStreamMaxVolume(AudioManager.STREAM_VOICE_CALL)
@@ -91,9 +95,7 @@ class MainActivity : TauriActivity() {
       val maxMusic = am.getStreamMaxVolume(AudioManager.STREAM_MUSIC)
       am.setStreamVolume(AudioManager.STREAM_MUSIC, maxMusic, 0)
 
-      Log.i(TAG, "audio state after: mode=${am.mode} speaker=${am.isSpeakerphoneOn} " +
+      Log.i(TAG, "volumes set: voiceVol=$maxVoice musicVol=$maxMusic (mode left at ${am.mode})")
-        "voiceVol=${am.getStreamVolume(AudioManager.STREAM_VOICE_CALL)}/$maxVoice " +
-        "musicVol=${am.getStreamVolume(AudioManager.STREAM_MUSIC)}/$maxMusic")
     } catch (e: Throwable) {
       Log.e(TAG, "configureAudioForCall failed: ${e.message}", e)
     }

desktop/src-tauri/src/android_audio.rs

@@ -57,11 +57,37 @@ fn audio_manager<'local>(
     Ok(am)
 }
 
+/// Set `AudioManager.MODE_IN_COMMUNICATION`. Call when a VoIP call starts.
+/// This tells the audio policy to route through the communication device
+/// path (earpiece/BT SCO) instead of the media path (speaker/BT A2DP).
+pub fn set_audio_mode_communication() -> Result<(), String> {
+    let (vm, activity) = jvm_and_activity()?;
+    let mut env = vm
+        .attach_current_thread()
+        .map_err(|e| format!("attach_current_thread: {e}"))?;
+    let am = audio_manager(&mut env, &activity)?;
+    // MODE_IN_COMMUNICATION = 3
+    env.call_method(&am, "setMode", "(I)V", &[JValue::Int(3)])
+        .map_err(|e| format!("setMode(MODE_IN_COMMUNICATION): {e}"))?;
+    tracing::info!("AudioManager: mode set to MODE_IN_COMMUNICATION");
+    Ok(())
+}
+
+/// Restore `AudioManager.MODE_NORMAL`. Call when a VoIP call ends.
+pub fn set_audio_mode_normal() -> Result<(), String> {
+    let (vm, activity) = jvm_and_activity()?;
+    let mut env = vm
+        .attach_current_thread()
+        .map_err(|e| format!("attach_current_thread: {e}"))?;
+    let am = audio_manager(&mut env, &activity)?;
+    // MODE_NORMAL = 0
+    env.call_method(&am, "setMode", "(I)V", &[JValue::Int(0)])
+        .map_err(|e| format!("setMode(MODE_NORMAL): {e}"))?;
+    tracing::info!("AudioManager: mode set to MODE_NORMAL");
+    Ok(())
+}
+
 /// Switch between loud speaker (`true`) and earpiece/handset (`false`).
-///
-/// Calls `AudioManager.setSpeakerphoneOn(on)` on the JVM. Requires that
-/// the audio mode is already `MODE_IN_COMMUNICATION` — MainActivity.kt
-/// sets this at startup, so by the time a call is up this is always true.
 pub fn set_speakerphone(on: bool) -> Result<(), String> {
     let (vm, activity) = jvm_and_activity()?;
     let mut env = vm

desktop/src-tauri/src/engine.rs

@@ -436,6 +436,16 @@ impl CallEngine {
         // hitting a "device busy" on some HALs.
         tokio::time::sleep(std::time::Duration::from_millis(50)).await;
 
+        // Set MODE_IN_COMMUNICATION right before audio starts — NOT at
+        // app launch. Setting it early hijacks system audio routing
+        // (music drops from BT A2DP to earpiece, etc.).
+        #[cfg(target_os = "android")]
+        {
+            if let Err(e) = crate::android_audio::set_audio_mode_communication() {
+                warn!("set_audio_mode_communication failed: {e}");
+            }
+        }
+
         let t_pre_audio = call_t0.elapsed().as_millis();
         if let Err(code) = crate::wzp_native::audio_start() {
             return Err(anyhow::anyhow!("wzp_native_audio_start failed: code {code}"));
@@ -1490,6 +1500,12 @@ impl CallEngine {
         #[cfg(target_os = "android")]
         {
             crate::wzp_native::audio_stop();
+            // Restore MODE_NORMAL so other apps' audio (music, etc.)
+            // routes normally again. Without this, the system stays in
+            // communication mode and BT A2DP music goes to earpiece.
+            if let Err(e) = crate::android_audio::set_audio_mode_normal() {
+                tracing::warn!("set_audio_mode_normal failed: {e}");
+            }
         }
     }
 }