ok then, get the rpm for your version here: http://rpm.livna.org/rlowiki/
install that and the new livna repo will be availiable, with lots of codecs and stuff. if you have other 3dr-party repos installed, they might conflict, do a search on the forums if you need more info on this.
then just do a "yum install gstreamer-plugins-good gstreamer-plugins-bad gstreamer-plugins-ugly" and an update. then most codecs should work fine in mplayer and the mplayer plugin. not sure about the real one, tho.