Well, in a decent video editor (Final Cut or Adobe Premiere or some other tool), the audio can be resync'd with the video. But if all you have is the AVI video quality to begin with... if it's really blocky, sounds like it's been extremely compressed.
Or put it differently, if all you have is the finished product (AVI file), it's a lot like having a burnt hamburger, sometimes. If you burn it, you can always make another hamburger if you still have some raw meat left (i.e. the original video source). But trying to rescue a badly burnt hamburger can be a difficult proposition even in best of circumstances.
A finished video file using a lossy compressed format can't really easily be 'reversed' or edited at the original quality -- because a lot of the original video data was lost when doing the inital compression and conversion into AVI format. Honestly, your
best bet is to have someone re-edit the original footage (if still available in its original form) with a modern video/audio editing tool and then once edited, convert into whatever format you like.
(This, BTW, is the same reason why movie and TV studios carefully store their original stock footage for years -- so they can later recopy, reedit, and reconvert into whatever modern-day format is needed with a minimum of any loss in quality.)
Wish I had better news. Sorry.
