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 get ["Xine"] and all the plugins for it.
See also: ["Xine"].