Try this:

var nLoadProgressInterval:Number;

_root.mcmpb.mpBar._xscale = 0;

function checkLoadProgress():Void {
mBytes = _root.sndAudio.getBytesLoaded()/_root.sndAudio.getBytesTotal();
_root.mcmpb.mpBar._xscale = mBytes;
trace(mBytes);
}


and on the button I have a call to the function like this

_root.track3.onRelease = function():Void {
nLoadProgressInterval = setInterval(checkLoadProgress, 100);
}