Sample code for Android AudioTrack Mixing

private void mixSound() throws IOException {
    AudioTrack audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC, 44100, AudioFormat.CHANNEL_OUT_STEREO, AudioFormat.ENCODING_PCM_16BIT, 44100, AudioTrack.MODE_STREAM);

    InputStream in1=getResources().openRawResource(R.raw.track1);      
    InputStream in2=getResources().openRawResource(R.raw.track2);

    byte[] music1 = null;
    music1= new byte[in1.available()]; 
    music1=convertStreamToByteArray(in1);
    in1.close();


    byte[] music2 = null;
    music2= new byte[in2.available()]; 
    music2=convertStreamToByteArray(in2);
    in2.close();

    byte[] output = new byte[music1.length];

    audioTrack.play();

    for(int i=0; i < output.length; i++){

        float samplef1 = music1[i] / 128.0f;      //     2^7=128
        float samplef2 = music2[i] / 128.0f;


        float mixed = samplef1 + samplef2;
        // reduce the volume a bit:
        mixed *= 0.8;
        // hard clipping
        if (mixed > 1.0f) mixed = 1.0f;

        if (mixed < -1.0f) mixed = -1.0f;

        byte outputSample = (byte)(mixed * 128.0f);
        output[i] = outputSample;

    }   //for loop
    audioTrack.write(output, 0, output.length);

}

 

Demo :