To play 3gp videos with audio using mplayer in Ubuntu Jaunty, add the following line to your /etc/apt/sources.list file
sudo wget http://www.medibuntu.org/sources.list.d/jaunty.list --output-document=/etc/apt/sources.list.d/medibuntu.list
You need the GPG key for medibuntu packages. Get it the following way.
sudo apt-get update
sudo apt-get install medibuntu-keyring
sudo apt-get update
now install the following packages
w32codecs mplayer mencoder amrnb amrwb
You should be able to hear the sound in mplayer now.
