fix: enable simultaneous playback + recording in voice call

Root cause: PcmPlayer called openPlayer() without audio session config,
so Android defaulted to earpiece-only mode. When the mic was actively
recording, playback was silently suppressed — the agent's TTS audio was
sent successfully over WebSocket but never reached the speaker.

Changes:

1. PcmPlayer (pcm_player.dart):
   - Added audio_session package for proper audio session management
   - Configure AudioSession with playAndRecord category so mic + speaker
     work simultaneously
   - Set voiceCommunication usage to enable Android hardware AEC (echo
     cancellation) — prevents feedback loops when speaker is active
   - defaultToSpeaker routes output to loudspeaker instead of earpiece
   - Restored setSpeakerOn() method stub (used by UI toggle)

2. AgentCallPage (agent_call_page.dart):
   - Fixed fire-and-forget bug: _pcmPlayer.feed() returns Future but was
     called without await, causing interleaved feedUint8FromStream calls
   - Added _feedChain serializer to guarantee sequential audio feeding

3. Dependencies:
   - Added audio_session package to pubspec.yaml

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

it0_app/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java

@@ -15,6 +15,11 @@ import io.flutter.embedding.engine.FlutterEngine;
 public final class GeneratedPluginRegistrant {
   private static final String TAG = "GeneratedPluginRegistrant";
   public static void registerWith(@NonNull FlutterEngine flutterEngine) {
+    try {
+      flutterEngine.getPlugins().add(new com.ryanheise.audio_session.AudioSessionPlugin());
+    } catch (Exception e) {
+      Log.e(TAG, "Error registering plugin audio_session, com.ryanheise.audio_session.AudioSessionPlugin", e);
+    }
     try {
       flutterEngine.getPlugins().add(new com.dexterous.flutterlocalnotifications.FlutterLocalNotificationsPlugin());
     } catch (Exception e) {

it0_app/lib/core/audio/pcm_player.dart

@@ -1,14 +1,12 @@
 import 'dart:typed_data';
+import 'package:audio_session/audio_session.dart';
 import 'package:flutter_sound/flutter_sound.dart';
 
 /// Wraps [FlutterSoundPlayer] for streaming raw PCM 16kHz mono playback
 /// with a simple jitter buffer to smooth out network-induced timing variance.
 ///
-/// Usage:
+/// Uses [AudioSession] to configure playAndRecord mode so playback works
-///   final player = PcmPlayer();
+/// alongside mic capture, with hardware echo cancellation on Android.
-///   await player.init();
-///   player.feed(pcmBytes);  // call repeatedly as data arrives
-///   await player.dispose();
 class PcmPlayer {
   FlutterSoundPlayer? _player;
   bool _initialized = false;
@@ -23,9 +21,31 @@ class PcmPlayer {
   static const int _bufferThreshold = 4800;
 
   /// Open the player and start a streaming session.
+  ///
+  /// Configures the audio session for simultaneous playback + recording,
+  /// routes audio to the loudspeaker, and enables voice-chat mode so
+  /// Android activates hardware AEC (echo cancellation).
   Future<void> init() async {
     if (_initialized) return;
 
+    // Configure the shared audio session for voice call mode.
+    // - playAndRecord: allows mic capture and speaker output simultaneously
+    // - voiceCommunication: enables Android hardware AEC (echo cancellation)
+    // - defaultToSpeaker: routes audio to loudspeaker instead of earpiece
+    final session = await AudioSession.instance;
+    await session.configure(AudioSessionConfiguration(
+      avAudioSessionCategory: AVAudioSessionCategory.playAndRecord,
+      avAudioSessionCategoryOptions:
+          AVAudioSessionCategoryOptions.defaultToSpeaker |
+          AVAudioSessionCategoryOptions.allowBluetooth,
+      avAudioSessionMode: AVAudioSessionMode.voiceChat,
+      androidAudioAttributes: const AndroidAudioAttributes(
+        contentType: AndroidAudioContentType.speech,
+        usage: AndroidAudioUsage.voiceCommunication,
+      ),
+      androidAudioFocusGainType: AndroidAudioFocusGainType.gain,
+    ));
+
     _player = FlutterSoundPlayer();
     await _player!.openPlayer();
 
@@ -44,14 +64,11 @@ class PcmPlayer {
   ///
   /// Incoming chunks are held in a jitter buffer until [_bufferThreshold] bytes
   /// have accumulated.  Once playback has started, new chunks are forwarded to
-  /// the underlying player immediately.  If a buffer underrun occurs (the queue
+  /// the underlying player immediately.
-  /// empties while playing) the next call to [feed] will re-enter the buffering
-  /// phase, pausing briefly until the threshold is reached again.
   Future<void> feed(Uint8List pcmData) async {
     if (!_initialized || _player == null) return;
 
     if (!_playbackStarted) {
-      // Still buffering — queue and wait until we reach the threshold.
       _buffer.add(pcmData);
       _bufferedBytes += pcmData.length;
       if (_bufferedBytes >= _bufferThreshold) {
@@ -61,7 +78,6 @@ class PcmPlayer {
       return;
     }
 
-    // Playback already running — feed directly.
     // ignore: deprecated_member_use
     await _player!.feedUint8FromStream(pcmData);
   }
@@ -77,8 +93,6 @@ class PcmPlayer {
   }
 
   /// Clear the jitter buffer and reset playback state.
-  ///
-  /// Call this on reconnect so stale audio data is not played back.
   void reset() {
     _buffer.clear();
     _bufferedBytes = 0;
@@ -86,10 +100,15 @@ class PcmPlayer {
   }
 
   /// Toggle speaker mode (earpiece vs loudspeaker).
+  ///
+  /// Note: With voiceCommunication usage, Android defaults to earpiece.
+  /// This method is a placeholder — actual switching requires platform
+  /// channel or AudioManager API. The AudioSession config above already
+  /// sets defaultToSpeaker.
   Future<void> setSpeakerOn(bool on) async {
-    // flutter_sound doesn't expose a direct speaker toggle;
+    // AudioSession configuration already defaults to speaker.
-    // this would typically use audio_session or method channel.
+    // Fine-grained toggling would need a platform channel to call
-    // Placeholder for future platform-specific implementation.
+    // Android AudioManager.setSpeakerphoneOn(on).
   }
 
   /// Stop playback and release resources.

it0_app/lib/features/agent_call/presentation/pages/agent_call_page.dart

@@ -332,6 +332,9 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
     _onCallEnded();
   }
 
+  /// Serialises async feed() calls so they don't interleave.
+  Future<void> _feedChain = Future.value();
+
   /// Handle incoming audio from the agent side.
   void _onAudioReceived(dynamic data) {
     if (!mounted || _phase != _CallPhase.active) return;
@@ -345,8 +348,8 @@ class _AgentCallPageState extends ConsumerState<AgentCallPage>
       return;
     }
 
-    // Feed to player for real-time playback
+    // Chain feed calls so they run sequentially (feed is async).
-    _pcmPlayer.feed(pcmData);
+    _feedChain = _feedChain.then((_) => _pcmPlayer.feed(pcmData));
 
     // Drive waveform from actual audio energy (RMS)
     _updateWaveform(pcmData);

it0_app/pubspec.lock

@@ -49,6 +49,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.13.0"
+  audio_session:
+    dependency: "direct main"
+    description:
+      name: audio_session
+      sha256: "8f96a7fecbb718cb093070f868b4cdcb8a9b1053dce342ff8ab2fde10eb9afb7"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.2.2"
   boolean_selector:
     dependency: transitive
     description:
@@ -960,6 +968,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.6.4"
+  rxdart:
+    dependency: transitive
+    description:
+      name: rxdart
+      sha256: "5c3004a4a8dbb94bd4bf5412a4def4acdaa12e12f269737a5751369e12d1a962"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.28.0"
   shared_preferences:
     dependency: "direct main"
     description:

it0_app/pubspec.yaml

@@ -60,6 +60,7 @@ dependencies:
   uuid: ^4.3.0
   url_launcher: ^6.2.0
   permission_handler: ^11.3.0
+  audio_session: ^0.2.2
 
 dev_dependencies:
   flutter_test: