I don't know if this will work for you or not...(my apologies if I get a little technical) but I found a problem with .avi files if you use more than one sound card.

I have a laptop, when I'm at home I used a SoundBlaster Audigy w/ 6.1 DTS EX. or Dolby Digital, but when I am out and about I use the internal sound card on my laptop. Usually 90% of my programs will just default to whichever card I am using, but for some reason with Windows Media Player if I don't go into the settings and designate the proper device I am using it will cause some problems with certain vids. It's kind-of weird, for the longest time I couldn't figure it out and it would piss me off, then I stumbled across the answer one day just playing around. Also with a lot of these avi files make sure you have downloaded the latest DIVX player, that usually takes care of the audio only without video issues.

Hope this helps...