Just a quick example:

class AudioRecorder
{
  int frequency = 8000;
  boolean keepGoing = false;
  android.media.AudioRecord audioRecord;
  int bufferSize;
  public short[] buffer;
  PApplet sketch;

  java.lang.reflect.Method updateBufferMethod;
  
  public AudioRecorder(PApplet _sketch) {
    sketch = _sketch;
    
    bufferSize = android.media.AudioRecord.getMinBufferSize(
      frequency, 
      android.media.AudioFormat.CHANNEL_CONFIGURATION_MONO, 
      android.media.AudioFormat.ENCODING_PCM_16BIT);
      
    audioRecord = new android.media.AudioRecord(
      android.media.MediaRecorder.AudioSource.MIC,
      frequency,
      android.media.AudioFormat.CHANNEL_CONFIGURATION_MONO,
      android.media.AudioFormat.ENCODING_PCM_16BIT,
      bufferSize);
      
    buffer = new short[bufferSize];

    // check to see if the host applet implements
    // public void updateBufferMethod(short[] buffer)
    try {
      updateBufferMethod = sketch.getClass().getMethod("updateBuffer", new Class[] { AudioRecorder.class });
    } catch (Exception e) {
      // just ignore
    }

  }

  public void stop() {
    keepGoing = false;
  }

  public void record() {
    keepGoing = true;				
    audioRecord.startRecording();
    new Thread(new Runnable() {
      public void run() {
        while (keepGoing) {
           audioRecord.read(buffer, 0, bufferSize);
           
           // Send to Processing sketch
           if (updateBufferMethod != null) {
             try {
               updateBufferMethod.invoke(sketch, new Object[] { this });
              } catch (Exception e) {
                System.err.println("Disabling updateBuffer() because of an error.");
                e.printStackTrace();
                updateBufferMethod = null;
              }           
           }
        }
        audioRecord.stop();
      }
    }).start();
  }

  short[] getBuffer() {
    return buffer;
  }
}

Main Sketch

AudioRecorder ar;
boolean recording = false;
public short[] buffer;

void setup() {
  ar = new AudioRecorder(this);
}

void draw() {
  
}

void mousePressed() {
  if (recording == false) {
    println("Going to record");
    ar.record();
    recording = true;
  } else {
    println("Stop recording");
    ar.stop();
    
    buffer = ar.getBuffer();
    for (int i = 0; i < buffer.length; i++) {
      println(buffer[i]);
    }
    
  }  
}

/*
void updateBuffer(AudioRecorder recorder) {
  println("update buffer");
  for (int i = 0; i < recorder.buffer.length; i++) {
      println(recorder.buffer[i]);
    }
}
*/