It means you shouldn't have moved your mouse...
But seriously, NPE is a very common error, very easy to get, so it is hard to answer without seeing the code.
Basically, it means you are trying to use an object that hasn't been initialized: the object is null. And here, it happens in your mouseMoved() function.

//global variables

import ddf.minim.*;
AudioPlayer player;
final static int highPoem = 5;
Minim[] minim = new Minim[highPoem];

//there is more code but this pertains to the audio


void mouseMoved() {
  minim = new Minim[highPoem];
  for (int j = 0; j < highPoem; j++) {
    for (int i = 0; i < topRow; i++) {
       boolean outside = false;
        if (mouseX > getImageX(i) &&
            mouseX < getImageX(i) + imagesTop[i].width &&
            mouseY > getImageY(i) &&
            mouseY < getImageY(i) + imagesTop[i].height) {

              outside = true;
              player = minim[j].loadFile("high" + j + ".aif", 1024);
              player.play();
       }
         if(outside) {
            player.close();
            minim[j].stop();
            super.stop();
         }
    }
  }
}

I see several problems in this code.
First, you load a sound file on mouseMoved, meaning you try to load it several times per second! You should load them in setup(): create the array there, and load it.
Likewise, why reset the minim[] array each time?
Beside, when you do new Minim[highPoem], you create the array, but not the objects. Perhaps that's your mistake. Because the NPE happens probably in the minim[j].loadFile() line: the array is full of nulls.

great, thanks for all of your help!

andrew

I'm doing a project similar to this one, by any chance did you figure out what was causing the NPE?

Well, it is great that you searched for a solution, but I suggest to re-read the whole topic attentively...
Particularly the "show your code" part (simplify it if it is too long), and the part about initializing an array.
See also the Why do I get a NullPointerException? article.

1. //Right now I'm using this code as a test to see if I can call different sounds randomly. I'm just not //exactly sure why my 'player' is not being called.
2. 
   
4. import ddf.minim.*;
5. 
   
6. int maxAudio = 2; // Total # of sounds
7. int audioIndex = 0; // Initial sound to be displayed is the first
8. 
   
9. AudioPlayer player;
10. Minim[] minim;
11. 
    
12. 
    
13. void setup() {
14.   size(200,200);
15.   minim = new Minim[maxAudio];
16. 
    
17. 
    
18.   
19.    for (int i = 0; i < maxAudio; i ++ ) {
20.    player = minim[i].loadFile("sound" + i + ".mp3", 2048);
21.    //player.play(); 
22.   }
23. }
24. 
    
25. 
    
26. 
    
27. void draw() {
28.   background(125);
29.   
30.   
31. }
32. 
    
33. void mousePressed() {
34.  
35.   audioIndex = int(random(minim.length));
36.   player.play();
37. }
38. 
    
39. 
    
40. 
    
41. void stop()
42. {
43.   // always close Minim audio classes when you are done with them
44.   player.close();
45.   minim[audioIndex].stop();
46.   
47.   super.stop();
48. }
49. 
    
50. 
    
 

since
minim[i]
is an array, shouldn't
player
be also an array, e.g. in line 9, 20, 36?

use it with audioIndex e.g.