Have you tried installing ffmpeg first?
-in synaptic, then try mplayer
If you ever want:
3gp movie wizard;This is a small perl/kommander script that helps you convert almost any
movie format to 3gp/mp4, suitable for watching on a modern mobile phones.It
should be extremely easy to use, yet thanks to the extremely powerful
mencoder and ffmpeg, offer many advanced features such as:........
Also in synaptic