I noticed this post and I thought correct in thinking that it was about AC3 audio. WELL HERE IS HOW TO MAKE IT WORK!.
All u need to do is get the AC3 audio filters, goto
http://www.doom9.net goto downloads, scroll down and click on the filters section, download the "Audiofilters - contains filters for AVI/AC3 and dual audio AVIs" file and then follow the instrustions cotained in the readme. This will work for both pre-win2k and winXP.
Hope this was a help.
P.S Check that the movie you are playing is useing AC3 audio. This will only fix AC3 audio codec playback.