It works fine on a windows box. And the thing is, I just converted a windows user to a Mac user, and now he's all PO'd cuz he can't play all AVI's. Even with DivX, VCL and Mplayer they don't all work. sucks.
The AVI in question is buggy and has incorrect headers, meaning that Quicktime can't tell that there is audio.
The Video played well with the 3ivx Codec from http://www.3ivx.com/ but DivX Doctor fell over whilst trying to doctor the AVI to something with sound.
See if your Windows Friend can fix it with VirtualDub. Apparently this Windows program can rewrite the headers.