Wait... it's an MP3, and the sound isn't playing? That qualifies as "weird". What player are you using, and have you tried others? It could be the player's fault...
Or, heck, I dunno, maybe Xvid deals with audio differently.
Download the Xvid codec, if you haven't already, hopefully it'll work...
Its .avi
I think that should work with VLC Media Player.
(
http://www.videolan.org/vlc/)
I always use VLC for my anime episodes, but I prefer to watch it on my Windows laptop because it's widescreen

I downloaded VLC player once..but it inverted all my colors..it was weird watching Naruto like that..it was even weirder to watch it in the right colors when the problem was solved. The problem was some codec I had downloaded was interfering with VLC.
But yea, the VLC player is really good