These are the steps in tried to get mplayer to play 3gp files with Audio in Ubuntu 8.04 (Under VirtualBox) and Ubuntu eee 8.04 on an Asus EEE 1000H.

First install the following lib

Search for these in the package manager and install them.

Next follow the instructions on this site.

A MPlayer icon should appear in your "Applications"->"Sound and Video". Click on that to run. If not you can start the player using the command "gmplayer" in your terminal.

When I run in Virtual Box it complains about "Error opening/initializing the selected video_out device".

Change it to X11 under Preferences->Video and you should be fine.