Passing var name into sound object

programming flash
1 

_global.myVar = "";

varSound = new Sound();
varSound.attachSound(_global.myVar);

function playSound(arg){
	arg.start(0,1);
}

sound_btn.onRelease = function(){
	playSound(varSound);
             //myVar traces out as sfx2 after clicking button in code below so i don't get why it wont play
	trace(_global.myVar);
}

I have the global myVar set as nothing to start. Within my movie a level deeper on click of a button i’m trying to set the global with this


preview_btn.onRelease = function(){
	//trying to set variable here, it traces out as sfx2
	_global.myVar = "sfx2";
	trace("i just updated the root var myVar to "+ _global.myVar);
}

But the sound doesn’t play when clicking sound_btn. So i don’t know if it has something to do with trying to pass a variable into the declaration of the sound object or what. Can someone help?