Just replace the /etc/apt/sources.list file with the one given by Chester, he's mentioned how to do it.
Then open the terminal and type
This will take some time cos new files available at these sites will be indexed.
Then again follow what he's said.
You can download whatever media player you like.
I prefer amaroK for audio, but on Ubuntu it will a huge download since it needs kdelibs. I guess it should be around 50MB, not sure though. You can check this up by typing
It will warn you about how much data to download.
For other purposes I prefer Mplayer or VLC.
Another good media player is xine with gxine as GNOME frontend.
XMMS is good as a winamp look alike but I don't really find it that good compared to above mentioned players.