Ubuntu does not support .rmvb files by default. Even though you may have installed the Gstreamer codec plugin and VLC (an audio player that supposedly able to play any media format), there is still no way that you can get the .rmvb file to play. There may be many tutorials out there that tell you to configure mplayer to play .rmvb file, but the easiest and fastest way to tackle this is simply install the Real Player for Linux.
Go to Real player for Linux website.
Download the deb package. (do not download the bin file)
Double click the downloaded deb file to install the Real Player 11 for Linux.
Once the installation is done, you should be able to play .rmvb or .rm files.
Right click on the .rmvb file and select “Open with Real Player 11”
To set Real Player as the default player for all .rmvb files, right click on any .rmvb file and select “Properties”
Go to the “Open With” tab.
Check “RealPlayer 11″ and click Close