Звук Flutter webrtc не работает на Android

Во Flutter я хочу сделать голосовой вызов между двумя узлами. Я использую Flutter-WebRTC. Я проводил некоторые тесты, и видео, кажется, работает с webrtc, но нет звука. Я вижу видео удаленного узла, но не слышу звука ни с одной из сторон.

Один пир — это мой телефон на Android, а другой — эмулятор.

Мой код main.dart:

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:flutter_webrtc/flutter_webrtc.dart';
import 'package:sdp_transform/sdp_transform.dart';
import 'dart:developer' as developer;

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'WebRTC lets learn together'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  bool _offer = false;
  RTCPeerConnection _peerConnection;
  MediaStream _localStream;
  RTCVideoRenderer _localRenderer = new RTCVideoRenderer();
  RTCVideoRenderer _remoteRenderer = new RTCVideoRenderer();

  final sdpController = TextEditingController();

  @override
  dispose() {
    _localRenderer.dispose();
    _remoteRenderer.dispose();
    sdpController.dispose();
    super.dispose();
  }

  @override
  void initState() {
    initRenderers();
    _createPeerConnection().then((pc) {
      _peerConnection = pc;
    });
    super.initState();
  }

  initRenderers() async {
    await _localRenderer.initialize();
    await _remoteRenderer.initialize();
  }

  void _createOffer() async {
    RTCSessionDescription description =
        await _peerConnection.createOffer({'offerToReceiveAudio': 1, 'offerToReceiveVideo': 1});
    var session = parse(description.sdp);
    print(json.encode(session));
    _offer = true;

    _peerConnection.setLocalDescription(description);
  }

  void _createAnswer() async {
    RTCSessionDescription description =
        await _peerConnection.createAnswer({'offerToReceiveAudio': 1, 'offerToReceiveVideo': 1});

    var session = parse(description.sdp);
    print(json.encode(session));

    _peerConnection.setLocalDescription(description);
  }

  void _setRemoteDescription() async {
    String jsonString = sdpController.text;
    dynamic session = await jsonDecode('$jsonString');

    String sdp = write(session, null);

    // RTCSessionDescription description =
    //     new RTCSessionDescription(session['sdp'], session['type']);
    RTCSessionDescription description =
        new RTCSessionDescription(sdp, _offer ? 'answer' : 'offer');
    print(description.toMap());

    await _peerConnection.setRemoteDescription(description);
  }

  void _addCandidate() async {
    String jsonString = sdpController.text;
    dynamic session = await jsonDecode('$jsonString');
    print(session['candidate']);
    dynamic candidate =
        new RTCIceCandidate(session['candidate'], session['sdpMid'], session['sdpMlineIndex']);
    await _peerConnection.addCandidate(candidate);
  }

  _createPeerConnection() async {
    Map<String, dynamic> configuration = {
      "iceServers": [
        {"url": "stun:stun.l.google.com:19302"},
      ]
    };

    final Map<String, dynamic> offerSdpConstraints = {
      "mandatory": {
        "OfferToReceiveAudio": true,
        "OfferToReceiveVideo": true,
      },
      "optional": [],
    };

    _localStream = await _getUserMedia();

    RTCPeerConnection pc = await createPeerConnection(configuration, offerSdpConstraints);
    pc.addStream(_localStream);

    pc.onIceCandidate = (e) {
      if (e.candidate != null) {
        print(json.encode({
          'candidate': e.candidate.toString(),
          'sdpMid': e.sdpMid.toString(),
          'sdpMlineIndex': e.sdpMlineIndex,
        }));
      }
    };

    pc.onIceConnectionState = (e) {
      print(e);
    };

    pc.onAddStream = (stream) {
      print('addStream: ' + stream.id);
      _remoteRenderer.srcObject = stream;
    };

    return pc;
  }

  _getUserMedia() async {
    final Map<String, dynamic> mediaConstraints = {
      'audio': false,
      'video': {
        'facingMode': 'user',
      },
    };

    MediaStream stream = await MediaDevices.getUserMedia(mediaConstraints);

    _localRenderer.srcObject = stream;

    return stream;
  }

  SizedBox videoRenderers() => SizedBox(
      height: 210,
      child: Row(children: [
        Flexible(
          child: new Container(
            key: new Key("local"),
            margin: new EdgeInsets.fromLTRB(5.0, 5.0, 5.0, 5.0),
            decoration: new BoxDecoration(color: Colors.black),
            child: new RTCVideoView(_localRenderer)
          ),
        ),
        Flexible(
          child: new Container(
              key: new Key("remote"),
              margin: new EdgeInsets.fromLTRB(5.0, 5.0, 5.0, 5.0),
              decoration: new BoxDecoration(color: Colors.black),
              child: new RTCVideoView(_remoteRenderer)),
        )
      ]));

  Row offerAndAnswerButtons() =>
      Row(mainAxisAlignment: MainAxisAlignment.spaceEvenly, children: <Widget>[
        new RaisedButton(
          onPressed: _createOffer,
          child: Text('Offer'),
          color: Colors.amber,
        ),
        RaisedButton(
          onPressed: _createAnswer,
          child: Text('Answer'),
          color: Colors.amber,
        ),
      ]);

  Row sdpCandidateButtons() =>
      Row(mainAxisAlignment: MainAxisAlignment.spaceEvenly, children: <Widget>[
        RaisedButton(
          onPressed: _setRemoteDescription,
          child: Text('Set Remote Desc'),
          color: Colors.amber,
        ),
        RaisedButton(
          onPressed: _addCandidate,
          child: Text('Add Candidate'),
          color: Colors.amber,
        )
      ]);

  Padding sdpCandidatesTF() => Padding(
        padding: const EdgeInsets.all(16.0),
        child: TextField(
          controller: sdpController,
          keyboardType: TextInputType.multiline,
          maxLines: 4,
          maxLength: TextField.noMaxLength,
        ),
      );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        body: Container(
            child: Column(children: [
          videoRenderers(),
          offerAndAnswerButtons(),
          sdpCandidatesTF(),
          sdpCandidateButtons(),
        ])));
  }
}

В build.gradle изменил minSdkVersion на 21.

В AndroidManifest.xml добавлено:

<uses-permission android:name="android.permission.INTERNET"/>
<uses-feature android:name="android.hardware.camera" />
<uses-feature android:name="android.hardware.camera.autofocus" />
<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" />

Я вижу видео удаленного узла, но не слышу звука ни с одной из сторон. Я что-то упускаю?