If you're using totem, you can select Edit->Preferences and a window called "totem-addons" will pop up. Grab the essential codec pack from MPlayer's website and extract it to totem-addons.
http://cambuca.ldhs.cetuc.puc-rio.br/xine/ a great place to see ?Xine information (RedHat version).
http://www.divx.com/divx/linux/ DivX for Linux - it has codecs that make Xine able to play .wmv and .avi files that have been encoded with DivX .
See also: ?Xine, Sound, Multimedia