OP, to see if I'm understanding you correctly, in a general sense (I know zip about Garageband or other machines of that nature), you have one track (Track 1) already recorded. You wish to lay another track on there but you need to hear the track playing while you lay a harmony voice on track 1, right? The one question I have is do you have a second track available to use? One thing is for sure. Whatever you are going to overdub will have be done with headphones on else the original track will bleed into the one you're recording the harmony on. My limited knowledge says there are only two ways to deal with that and one of 'em is seriously chancy: a) sing the entire song and hope real hard you don't blow a a single note 2) wear earphones and record onto a second track, but don't allow the machine to 'hear' the first track as you record, just the second track. Think of the situation as you would if recording your voice to a track but listening to a drum track (to keep rhythm). When the cut is finished, because you weren't recording the drum track, just hearing it, it won't be there when you listen to the finished product.