well ubuntu out of the box doesn’t support amr sounds often found in mobile phone videos. the problem is that ffmpeg isn’t compiled with amr support… medibuntu has a version compiled with amr support, so….

add a medibuntu repository file:
sudo wget http://www.medibuntu.org/sources.list.d/hardy.list -O /etc/apt/sources.list.d/medibuntu.list

this creates a new source list in the sources.list.d folder and leaves your main source file unchanged.

then also add the GPG key:
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update

this is also described on the medibuntu page.

you should get an update notice and if not it should be sufficient to update the libavcodec1d (ffmpeg codec library).

now your mobile phone 3gp videos should play with sounds.