I'd suggest extracting the audio, 10 GB is a pain in the ass to edit. Personally I prefer synching with Samplitude (multitrack hd recording app), cause its graphic style makes synching quite easy. Sometimes, when the two audio sources aren't quite in synch and some cutting needs to be done, Samplitude is killer.

You could also make a DVD out of your AVI file as if the audio was perfect, then from the finished DVD, de-mux the audio, resynch it with the new source, then re-mux it. Then you'd have your 10 GB AVI off the table and are able to work with handy files. As the video remains untouched when you mux, this is probably the way for you to go.