Just like with colors, numbers, images and basically any object, you can use an array or an arrayList.

Code example:

1. import ddf.minim.*;
2. AudioPlayer [] players = new AudioPlayer[2]; // choose any number
   
3. Minim minim;
4. 
   
5. void setup() {
6.   size(512, 250, P2D);
7.   minim = new Minim(this);
8.   players[0] = minim.loadFile("fileOne.mp3"); // then load the files
   
9.   players[1] = minim.loadFile("fileTwo.mp3");
10.   for (int i=0; i<players.length; i++) {
11.     players[i].play();
12.   }
13. }
14. 
    
15. void draw() {
16.   background(0);
17.   displayAudio(players[0], 50, color(0,0,255));
    
18.   displayAudio(players[1], 150, color(255,0,0));
19. }
20. 
    
21. // you can have generalized functions to prevent repeating similar code
    
22. void displayAudio(AudioPlayer player, int playerHeight, color c) {
23.   stroke(c);
24.   for(int i=0; i<player.left.size()-1; i++) {
25.     line(i, playerHeight + player.left.get(i)*50, i+1, playerHeight + player.left.get(i+1)*50);
26.     line(i, playerHeight + 25 + player.right.get(i)*50, i+1, playerHeight + 25 + player.right.get(i+1)*50);
27.   }
28. }
29. 
    
30. void stop() {
31.   for (int i=0; i<players.length; i++) {
32.     players[i].close();
33.   }
34.   minim.stop();
35.   super.stop();
36. }

I tried and it works.