messed-up avi sound

Hey, guys. I was hoping you could help me out with this. I've tried downloading certain TV episodes using Limewire and Myster. Both times whenever I picked an avi file the sound was gone, with maybe a half-second of sound an the start. I then used Myster to start a peer-to-peer connection with my roommate's PC as a test. This time I got sound, but things were so screwed up the sound was always moving faster than the video, getting more incompreensible as the file played on. Anyone have any ideas what the general problem is? It's really annoying me to download these files and not to be able to watch them.