The problem is your code runs everytime an event occurs on the button (regardless check or uncheck)
You need to check the button status like this:
public void actionPerformed (ActionEvent event)
{
Object source = event.getSource();
if(source==b1){
if(b1.isSelected()){
music[1].play();
}
else{
music[1].stop()
}
}
}
You should also consider stopping the songs with .stop() rather then pointing the a null song.