使用Android的底层AudioTrack和AudioRecord类来实现VoIP，可以通过自定义包格式和协议来转移媒体数据。以下是一个简单的示例代码：

private AudioTrack mAudioTrack;
private AudioRecord mAudioRecord;

// set up audio track
int minBufferSize = AudioTrack.getMinBufferSize(16000, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT);
mAudioTrack = new AudioTrack(AudioManager.STREAM_VOICE_CALL, 16000, AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT, minBufferSize, AudioTrack.MODE_STREAM);

// set up audio record
int maxBufferSize = AudioRecord.getMinBufferSize(16000, AudioFormat.CHANNEL_IN_MONO, AudioFormat.ENCODING_PCM_16BIT);
mAudioRecord = new AudioRecord(MediaRecorder.AudioSource.MIC, 16000, AudioFormat.CHANNEL_IN_MONO, AudioFormat.ENCODING_PCM_16BIT, maxBufferSize);

// start recording and playing
mAudioRecord.startRecording();
mAudioTrack.play();

// read data from audio record and write to audio track
byte[] buffer = new byte[1024];
while (true) {
    int numBytes = mAudioRecord.read(buffer, 0, buffer.length);
    mAudioTrack.write(buffer, 0, numBytes);
}

此示例代码实现了从麦克风输入到扬声器输出的实时音频流。将其与自定义的VoIP协议配合使用即可实现VoIP通信。