I actually play almost all .avi files using Quicktime (and yes, sound and video both work), I only very rarely come across one that won't work, but then other programs tend to have issues with them too. You just need to make sure that you have the appropriate Quicktime plugins, and you'll be fine. I was going to attach the files you need, but they are too big for MacRumors (limit is only 100kb), but if you PM me your email address I can send them to you.
The plugins I use are Divx, 3ivx, and an AC3 audio plugin. Those 3 seem to handle everything. There are a couple other plugins in my Quicktime folder, but I don't remember putting them there, or what they are for, so I don't think they are important. Just download the above 3 and you should be good to go.