//Shortest program to just play two sinus waves with Android

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import android.media.AudioTrack;
import android.media.AudioFormat;
import android.media.AudioManager;

import android.media.AudioRecord;
import android.media.MediaRecorder;

Sound sound;

public void setup()
{
    sound = new Sound();
    fullScreen();
    orientation(LANDSCAPE);
}

public void draw()
{
    background(0,0,255);
}

public class Sound implements Runnable
{
    private AudioTrack audioTrack; //Connection to sound card
    private int sr=44100; //Sample rate
    private int buffsize=512; //buffer size
    private int buffsize2=buffsize*2;
    private ScheduledExecutorService schedExecService; //realtime process    
    private short[] shortbuffer = new short[buffsize*2]; //stereo buffer l0 r0 l1 r1 l2 r2 ... frequently sent to sound card

    private double t=0.0; //realtime
    private double dt = 1.0/(double)sr; //time step according to sampling rate.
    private float MAX_SHORT = 32767.0;
    public Sound()
    {
        try
        {                                                
            audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC, sr, 
                                    AudioFormat.CHANNEL_OUT_STEREO, 
                                    AudioFormat.ENCODING_PCM_16BIT, 
                                    buffsize*10, 
                                    AudioTrack.MODE_STREAM);
            audioTrack.setStereoVolume(1.0f, 1.0f);
            
            audioTrack.play();            
        }
        catch(Exception eee)
        {
            System.out.println("FEHLER: "+eee);
        }     
        
        schedExecService = Executors.newSingleThreadScheduledExecutor();
        long period = (buffsize*1000)/sr; //Seconds per Beat==60/BPM, die Hälfte weil 8tel, mal 1000 weil Millisekunden.
        schedExecService.scheduleAtFixedRate(this, 0, period, TimeUnit.MILLISECONDS);                
        
        
    }
    public void run()
    {
        for(int i=0;i<buffsize2;i+=2)
        {
             //This may be the part of the programm where you might want to modify something like
             //introducing other sound sources / directional sound and so on:
             float left  = MAX_SHORT*sin(TWO_PI*440.0*(float)t);
             float right = MAX_SHORT*sin(TWO_PI*770.0*(float)t);
             shortbuffer[i]=(short)left;
             shortbuffer[i+1]=(short)right;
             t+=dt; //increment of realtime for each sample
        }
      
        audioTrack.write(shortbuffer, 0,buffsize2);      
    }
}

Code 0-1: Androidsound001