AudioManager audioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE);
 private void setSpeakerphoneOn(boolean on) {
                 if(on) {
                         audioManager.setSpeakerphoneOn(true);
                 } else {
                         audioManager.setSpeakerphoneOn(false);//關閉揚聲器
                         audioManager.setRouting(AudioManager.MODE_NORMAL, AudioManager.ROUTE_EARPIECE, AudioManager.ROUTE_ALL);
                         setVolumeControlStream(AudioManager.STREAM_VOICE_CALL);
                         //把聲音設定成Earpiece（聽筒）出來，設定為正在通話中
                         audioManager.setMode(AudioManager.MODE_IN_CALL);
                 }
 }
 

static final int bufferSize = 200000;
final short[] buffer = new short[bufferSize];
short[] readBuffer = new short[bufferSize];
public void run() {
     isRecording = true;
     android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_URGENT_AUDIO);
     int buffersize = AudioRecord.getMinBufferSize(11025, AudioFormat.CHANNEL_CONFIGURATION_MONO, AudioFormat.ENCODING_PCM_16BIT);
     arec = new AudioRecord(MediaRecorder.AudioSource.MIC, 11025, AudioFormat.CHANNEL_CONFIGURATION_MONO, AudioFormat.ENCODING_PCM_16BIT, buffersize);
     atrack = new AudioTrack(AudioManager.STREAM_VOICE_CALL, 11025, AudioFormat.CHANNEL_CONFIGURATION_MONO, AudioFormat.ENCODING_PCM_16BIT, buffersize, AudioTrack.MODE_STREAM);
     atrack.setPlaybackRate(11025);
     byte[] buffer = new byte[buffersize];
     arec.startRecording();
     atrack.play();
           while(isRecording) {
               arec.read(buffer, 0, buffersize);
               atrack.write(buffer, 0, buffer.length);
               }
     } 
 

3. PS
http://mindtherobot.com/blog/624/android-audio-play-an-mp3-file-on-an-audiotrack/