Oops, I didn't mean to type that "if" in the last sentence of my previous comment. It changes the meaning from what I intended, but now I can't edit it. Ah well.

QT Sync is a handy app and I've used it for a while, but I just learned that VLC can do this as well, albeit in a not so obvious way. While playing the video, press F or G to slow down or speed up the audio, respectively. There's also an option hidden in the advanced preferences. So if you could just use VLC if you already have it.

More info

Very useful for a simple play (correcting the audio) or even to save the new audio with fixed audio sync.

Works just fine in Snow Leopard (10.6.1)

Love it! Had a movie in MKV, ~4GB, that wasn't in sync with the audio. After some playing with the app I found out the duration of the video was about 17 min less than it should. After some trials and errors I managed to get it right.
It has the same saving options as QuickTime (self-containter, as a reference, export to...)

Small note: I have Perian installed.