Divx Player? I'd stop using that. To my mind there are a few ace players that you should use:
1) Media Player Classic - This is not Windows media player, but an open source very lightweight piece of software that looks as simple as the old version of the windows media player, but shares only looks. You can play rm / mov, and pretty much everything in this player.
2) VLC - Can find out and normally sort out codec problems
3) Zoomplayer - Can use different filters from different players, ie. audio from WinDVD and video from PowerDVD to make it the best option for viewing DVDs/VCDS
As for the specific codec problem, divx player is meant to say what geezus said, you need an ac3 codec. Grab the pack or you can get the AC3 filter only from here:
http://forum.dead-donkey.com/viewtopic.php?t=5640