[Alex T]

если я правильно понял:

var tracks:Array=["track1","track2","track3","track4","track5"];
_global.numCurTrack=0;

prev_but.onPress=function():Void {
	if(_global.numCurTrack==0) _global.numCurTrack=tracks.length-1;
		else _global.numCurTrack--;
	trace(tracks[_global.numCurTrack]);
}

next_but.onPress=function():Void {
	if(_global.numCurTrack==tracks.length-1) _global.numCurTrack=0;
		else _global.numCurTrack++;
	trace(tracks[_global.numCurTrack]);
}