Hi, I sovled this same kinda problem with my system, You probably have also by now,
But just in case anyone looks for an answer, this is what I did
My system is usually from the unstable/development cycle, Cauldron,
But currently I am upto mga3 release and updated.
At first I could not play mpeg movies or flac tunes. (this could be particular)
I updated the repos and made sure to select all but debug and testing.
Now I was able to select the plugins for my players, mplayer needs mencoder and avidemux is needed too.
gstreamers ugly collection, vlcś plugins, ffmpeg and dvdcss-devel. Oh, and lame.
hope this helps others with the mga3 install.