//In whatever class you have the sound play
private boolean soundHasPlayed = false;
private GreenfootSound mySound;

//Constructor and other methods
...

public void act()
{
    if (!soundHasPlayed)
    {
        mySound.play();
        soundHasPlayed = true;  //This will cause this if block never to run again, unless you have code somewhere that turns it to false again.
    }
    ...
}

private boolean soundhasplayed;
// in act
if(!soundhasplayed)
{
    playintro();
    while (isintrostillplaying) delay(1); // this pauses everything
    soundhasplayed = true;
}

boolean hasintroPlayed = false;
        {
            if (!hasintroPlayed)  
             {  
                 {
                     playIntro();  
                     while (IntroStillPlaying())  
                     hasintroPlayed = true;  
                    }
                }  
        }

public void act()
boolean hasintroPlayed = false;  
 
        {  
            if (!hasintroPlayed)    
             {    
                 {  
                     playIntro();    
                     while (IntroStillPlaying())    
                     hasintroPlayed = true;    
                    }  
                }    
        }