MediaRecorder 录制通话中远端声音

使用 Android 的 MediaRecorder 接口只能录制本地的音频，无法录制远端的音频。如果你想要录制通话中远端的声音，可以考虑使用 WebRTC 的 AudioTrack 接口。

具体来说，可以在 PeerConnection 的回调函数中获取远端的音频数据，然后写入到一个本地文件中。以下是一个简单的示例代码，仅供参考：

// 初始化 PeerConnectionFactory
PeerConnectionFactory.initialize(PeerConnectionFactory.InitializationOptions.builder(context).createInitializationOptions());
PeerConnectionFactory factory = PeerConnectionFactory.builder().createPeerConnectionFactory();

// 创建 PeerConnection
PeerConnection peerConnection = factory.createPeerConnection(new PeerConnection.Observer() {
    @Override
    public void onSignalingChange(PeerConnection.SignalingState signalingState) {}
    @Override
    public void onIceConnectionChange(PeerConnection.IceConnectionState iceConnectionState) {}
    @Override
    public void onIceConnectionReceivingChange(boolean b) {}
    @Override
    public void onIceGatheringChange(PeerConnection.IceGatheringState iceGatheringState) {}
    @Override
    public void onIceCandidate(IceCandidate iceCandidate) {}
    @Override
    public void onIceCandidatesRemoved(IceCandidate[] iceCandidates) {}
    @Override
    public void onAddStream(MediaStream mediaStream) {
        // 获取远端的音频轨道
        AudioTrack audioTrack = mediaStream.audioTracks.get(0);
        audioTrack.setEnabled(true);
        audioTrack.setVolume(100);

        // 创建音频文件
        File audioFile = new File("/sdcard/test.pcm");
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(audioFile);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        // 每秒钟读取一次音频数据，并写入文件
        final int SAMPLE_RATE = 44100;
        final int BUFFER_SIZE = SAMPLE_RATE;
        final byte[] buffer = new byte[BUFFER_SIZE];
        Timer timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                int bytesRead = audioTrack.read(buffer, 0, BUFFER_SIZE);
                try {
                    fos.write(buffer, 0, bytesRead);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }, 0, 1000);
    }
    @Override
    public void onRemoveStream(MediaStream mediaStream) {}
    @Override
    public void onDataChannel(DataChannel dataChannel) {}
    @Override
    public void onRenegotiationNeeded() {}
    @Override
    public void onAddTrack(RtpReceiver rtpReceiver, MediaStream[] mediaStreams) {}
}, new PeerConnection.RTCConfiguration(Collections.emptyList()));

// 发起呼叫
MediaConstraints constraints = new MediaConstraints();
constraints.mandatory.add(new MediaConstraints.KeyValuePair("OfferToReceiveAudio", "true"));
peerConnection.createOffer(new SdpObserver() {
    @Override
    public void onCreateSuccess(SessionDescription sessionDescription) {
        peerConnection.setLocalDescription(new SdpObserver() {
            @Override
            public void onCreateSuccess(SessionDescription sessionDescription) {}
            @Override
            public void onSetSuccess() {}
            @Override
            public void onCreateFailure(String s) {}
            @Override
            public void onSetFailure(String s) {}
        }, sessionDescription);
    }
    @Override
    public void onSetSuccess() {}
    @Override
    public void onCreateFailure(String s) {}
    @Override
    public void onSetFailure(String s) {}
}, constraints);

// 等待呼叫结束
Thread.sleep(10000);

// 停止录制
timer.cancel();
audioTrack.release();
peerConnection.close();

需要注意的是，上述代码仅作为参考，实际使用时可能需要进行更多的错误处理和参数调整。另外，由于网络延迟等原因，远端音频数据可能会有一定的延迟，需要进行相应的处理。