Io ho fatto così:
codice:
stop();
musica = new Sound();
musica.loadSound("loop.mp3", false);

entratesto.text = "loading";
this.onEnterFrame = function() {
	entratesto._visible = !entratesto._visible;
	if(musica.getBytesLoaded() > 1024 && musica.getBytesLoaded() == musica.getBytesTotal()){
		delete this.onEnterFrame;
		nextFrame();
	}
};