I'm not sure this will work, as I haven't tested it and I don't understad your script well. But it would be something like this:

Code:
onEnterFrame=function(){
if(Target1.onTarget == true && Target2.onTarget == true && Target3.onTarget == true ){
applause = new Sound();
applause.attachSound("applause");
applause.start(0,1);
}
}
The onEnterFrame will keep running until all the onTarget variables become true in all the target movieclips or buttons, then will load and start the applause sound. You need to import the applause sound into the library, export it for actionscript and assign an Identifier name "applause". Hope that helps.