This is probably of no help to you, but, first, just optimize the code a bit, move your loadImage block out of draw() and up to setup() so the images won't flicker and it won't slow down your code to reload images each draw cycle.

Second, and here is where I can probably be of no help, I recall seeing something where minim can only play one sound at time.  To play the next sound, you have to stop the first sound.  To get it to play different files at the same time, you have to setup audio channels for each channel of sound.  It might be like:

Minim minim;
Minim minimbass;
Minim minimcybal;
Minim scream;

Or, it might be like Minim minim = new minim(CHANNEL,BASS) or something else entirely.  Of course, once you set up the multiple Minim 's or channels or whatever, then you have to reference them within their Minim or channel reference in your draw() and functions .. eg apply the stops() and starts() for each on MousePressed.

Also, probably also unless info, review the other mousevent options besides mousePressed(), I mean there are ways to detect mousereleased and held down for example.  Like the way it's written, the sound1.play(); sound2, sound3, sound4 are only part of the draw cycles while the mouse is held down and that might not be the best way to implement play();

SEARCH for minim channel and search for minim start() stop() two sounds.

mousePressed() must not be called, it is called by Processing. And, yes, don't use loadXxx() in draw().

Hi all,
    thanks for your help. I got some help at school and found a couple of issues that was causing the problem. One was that I wasn't using the rewind function. The other was that I needed to include the isPlaying function.

Here is the updated code:

import librarytests.*;
import org.openkinect.*;
import org.openkinect.processing.*;
import hypermedia.video.*;
import processing.video.*;
import ddf.minim.*;
import ddf.minim.signals.*;
import ddf.minim.analysis.*;
import ddf.minim.effects.*;

/*
To do:
video image background
rollover states for sound graphics, eg: glow when initial threshold is reached
multiple thresholds






*/
   PImage bass;
   PImage organ;
   PImage cymbal;
   PImage scream;
  
  Capture video;
  PImage backgroundImage;

Minim minim;

AudioPlayer sound1;
AudioPlayer sound2;
AudioPlayer sound3;
AudioPlayer sound4;
 

void setup(){
   size(600,600);
   background(0, 0, 0);
bass = loadImage("bass.png");
organ = loadImage("organ.gif");
scream = loadImage("scream.gif");
cymbal = loadImage("cymbal.gif");
  


minim = new Minim(this); 
sound1 = minim.loadFile("bass.mp3");
sound2 =  minim.loadFile("organ.wav");
sound3 =  minim.loadFile("bass.wav");
sound4 = minim.loadFile("scream.wav");

}

void draw()
  {
  image(bass, 0, 0);
  image(organ, 0, 150);
  image(scream, 320, 0);
  image(cymbal, 0, 0);
 
  // if a sound has stopped then we should rewind it
  println (sound1.isPlaying() + "  " + sound1.position() + "  " + sound1.length());
 
  if (sound1.isPlaying() == false)
    {
    sound1.rewind();
    }
   
      if (sound2.isPlaying() == false)
    {
    sound2.rewind();
    }
   
      if (sound3.isPlaying() == false)
    {
    sound3.rewind();
    }
   
      if (sound4.isPlaying() == false)
    {
    sound4.rewind();
    }
 
  }

void mousePressed()
  {
 
   if(mouseX > 0 && mouseX < width/2 && mouseY > 0 && mouseY < 600)
     {
     sound1.play();
     }      
   if(mouseX > width/2 && mouseX < 600 && mouseY > 0 && mouseY < height/2)
  {
    sound2.play();
  }  
if(mouseX > 0 && mouseX < width/2 && mouseY > height/2 && mouseY < 600)
   {
     sound3.play();
   }   
if(mouseX > width/2 && mouseX < 600 && mouseY > height/2 && mouseY < 600)
    {
      sound4.play();
    }    
 }
  

void stop()
{
  // always close Minim audio classes when you are finished with them
  sound1.close();
  sound2.close();
  sound3.close();
  sound4.close();
 
  //isplaying
  // always stop Minim before exiting
  minim.stop();
 
  super.stop();
}