Good Night,

If you are having problems to find the codecs to play RMVB. Use:

$ sudo apt-get update

For me, just this, update the sourcelist and now I can find the codecs.

Best Regards,
Matheus

Hello,

How to fix Rhythmbox to play Mp3?

First of all remove it with:

$ sudo apt-get remove rhythmbox –purge

Update your sourcelist:

$ sudo apt-get update

Install o Rhythmbox again:

$ sudo apt-get install rhythmbox

UPDATE: Maybe you don’t need to remove your Rhythmbox, just update the sourcelist and try to play. Then it will try to find the codecs. I think this because it solved my RMVB problem.

Best Regards,
Matheus

Reference:
Ubuntu Geek