It'd help a lot if you had also told us where the error occurs!

I'm so sorry but I can't understand your code at all!  

There are these 2 classes -> first & second_
and I can't find them anywhere!

I had implemented a function the other day which pauses/resumes the music when mouse is clicked.

I believe if you modify the code below to use both mousePressed() & mouseReleased(),
you'll accomplish what you want!

Take a look:
http://forum.processing.org/topic/draw-to-off-screen-buf-render-it-to-image-move-image-with-everything-drawn-on-it-doesn-t-work

void mousePressed() {
  if ( song.isPlaying() ) {
    song.pause();
    noLoop();
  }

  else {
    song.play();
    loop();
  }
}

Sorry about that.

I updated the code with the one you gave me.The problem I have now is that when I release the mouse and it stops the non-looped sound, the sound doesn't appear when I click the mouse again.

What I want is the audio clip to loop when the mouse is clicked since its only an 8 second sound and when the user releases their mouse I want the sound to stop. I want this function to happen any time the user clicks and releases their mouse.

1. first circle_;
   second_[] smallcircle = new second_[4000];
   
   import ddf.minim.*;
   Minim minim;
   AudioPlayer player;
   
   // Knob Features
   boolean knobHighlight1 = false;
   boolean knobHighlight2 = false;
   float knob1, knobOne, knob2, knobTwo;
   int xKnob1;
   int yKnob1;
   int yKnob2;
   int knobMass;
   PImage bg;
   
   
   void setup() {
   size(640,480);
   minim = new Minim(this);
   player = minim.loadFile("bubbles.wav");
   
   textSize(30);
   text("Fill My World With Colour",50,50);
   textAlign(CENTER,CENTER);
   
   circle_ =new first ();
   
   for (int s = 0; s <100; s++) {
     circle_ = new first();
     smallcircle[s]=new second_();
   }
   
   knob1 = color(350);
   knobOne = color(230);
   knob2 = color(350);
   knobTwo = color(230);
   xKnob1=470;
   yKnob1=10;
   yKnob2=40;
   knobMass=10;
   
   }
   
   void stop(){
     minim.stop();
     super.stop();
   }
   
   void draw() {
   update(mouseX, mouseY);
   
     // Knobs Are Lit With Colour When Highlighted By Mouse
     if (knobHighlight1) {
       fill(knob1);
             
       knob1pressed(); // Function For Chosen Knob
     }
     else {
       fill(knobTwo);
     }
     ellipse (xKnob1, yKnob1, knobMass, knobMass);
   
     if (knobHighlight2) {
       fill(knob2);       
   
       knob2pressed();
     }
     else {
       fill(knobTwo);
     }
     ellipse (xKnob1, yKnob2, knobMass, knobMass);
   }
   
   void mousePressed(){
     if ( player.isPlaying() ) {
       player.pause();
       noLoop();
      
   }
   
   else {
     player.play();
     loop();
   }
   }
   
   // Boolean To Cause Action Once Mouse Is Pressed
   void update(int x, int y) {
     if ( knobHighlight1(xKnob1, yKnob1, knobMass)) {
       knobHighlight1 = true;
       knobHighlight2 = false;
     }
     else if ( knobHighlight2(xKnob1, yKnob2, knobMass) ) {
       knobHighlight1 = false;
       knobHighlight2 = true;
     } 
   }
   
   // Position of the Knobs
   boolean knobHighlight1(int x, int y, int diameter) {
     float xRange = x - mouseX;
     float yRange = y - mouseY;
     if (sqrt(sq(xRange) + sq(yRange)) < 15 ) {
       return true;
     }
     else {
       return false;
     }
   }
   
   
   boolean knobHighlight2(int x, int y, int diameter) {
     float xRange = x - mouseX;
     float yRange = y - mouseY;
     if (sqrt(sq(xRange) + sq(yRange)) < 15 ) {
       return true;
     }
     else {
       return false;
     }
   }
   
   void knob1pressed() {
     if (mousePressed==true) {
       fill(0);
       circle_.draw_();
       noStroke();
       fill(random(0, 255));
     }
   }
   void knob2pressed() {
     if (mousePressed==true) {
       for (int s = 0; s < 50; s++) {
         fill(0);
         smallcircle[s].draw_();
         smallcircle[s].move();
       }
     }
   }
   
   

I think when you issue a au_player1.close(); you simply destroy the referenced AudioPlayer object!
And when you try to play it again, you got yourself a null reference, since it doesn't exist anymore!

Instead of use close(), use au_player1.pause(); to stop the sound.

Here's how I did that here:

void mousePressed() {
  au_player1.loop();
}

void mouseReleased() {
  au_player1.pause();
}

P.S.: There's a redundancy in your code inside the for s loop block within setup():

You create 100 first objects! However, variable circle_ can only store 1 of them!
Since it's neither an array, nor a Collection!

This worked perfectly! Thank you so much!