Ubuntu and audio drivers

I have 700mhz 128mb of ram PC which I finally dropped windows on and Instaleld Ubuntu version 5.04 on it. The install went great and ubuntu runs quite well. In fact I am rather happy with it. I have one problem and that is I am unable to get mp3's to play in the totem movie player or any other sound player. It tells me that I am missing the plugin. It will play audio cd's however not mp3's. How would I rectify this situation?