OK, so you want to use your Mplayer to play all the media files, including the famous RMVB? Follow this commands:
sudo gedit /etc/apt/sources.list
add the following lines:
deb http://archive.ubuntu.com/ubuntu intrepid universe multiverse
deb-src http://archive.ubuntu.com/ubuntu intrepid universe multiverse
sudo apt-get update
sudo wget http://www.medibuntu.org/sources.list.d/intrepid.list --output-document=/etc/apt/sources.list.d/medibuntu.list
sudo apt-get update
sudo apt-get install medibuntu-keyring
sudo apt-get update
sudo apt-get install w32codecs libdvdcss2
Now, open up your Mplayer, right click on the video window, go to preferences:
Select the Video tab and change the Available drivers: to x11 X11 (XImage/Shm).
Select the Codecs & demuxer tab and change the Video codec family: to RealVideo decoder and the Audio codec family: to FFmpeg/libavcodec audio decoders. When you’re done, click OK and close down MPlayer.
You’re ready to go. Open an rmvb file with mplayer, it should do the trick