SOUND DURATION

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class StartGame here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class StartGame extends World
{
    public GreenfootSound start = new GreenfootSound("Background Music.mp3");
    /**
     * Constructor for objects of class StartGame.
     * 
     */
    public StartGame()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(800, 600, 1); 
        prepare();
    }
    
    public void act(){
        start.play();
        //stopMusic();
    }
    
    /*public void stopMusic(){
         if(Greenfoot.mouseClicked(Play.class)){
             start.stop();
    }
} */
    /**
     * Prepare the world for the start of the program.
     * That is: create the initial objects and add them to the world.
     */
    private void prepare()
    {
        Start start = new Start();
        addObject(start,389,363);
        start.setLocation(412,346);
        Help help = new Help();
        addObject(help,421,407);
        help.setLocation(412,395);
    }
}

Here is the code for the Button (Play actor)

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

public class Play extends Actor
{
    public Play()
    {
        GreenfootImage image = getImage();
        int myNewHeight = (int)image.getHeight() / 2;
        int myNewWidth = (int)image.getWidth() / 2;
        image.scale(myNewWidth, myNewHeight);
    }
    
    public GreenfootSound start = new GreenfootSound("Background Music.mp3");
    public void act() 
    {
        click();      
        start.stop();
    }
    
      private void click()
   {
      if(Greenfoot.mouseClicked(this)){
          Greenfoot.setWorld(new RealGame());
        }
}
}