I have used this commad in Ubuntu 10.10 and should work on every ubuntu versions.
$ ffmpeg -i commedy_2.flv -ar 22050 comedy_2.mp4
If you don’t have ffmpeg package, Install it using command
sudo apt-get install ffmpeg
Remembers files converted this way may or may not work for some mobiles. For that you will need to tweak the 22050 portion and set something that will be played by your mobile phones. It is usually bit rate.
sources : linuxquestions.org