Codex help?

[Alison]

I'm not sure what I need here. But I downloaded a movie from a Bittorrent site.
That was fiine, it plays fine, nice clear picture, but I have no sound. Doesn't matter what player I use, still no sound.
Its a Avi file movie. windows says I need a codex, but it can't find it.
something about four cc indentifier. The sound file for this movie is "ac3".
Does anyone know what I'm talking about?
Thanks Alison