To convert the audio from stereo to mono without changing the video part, you can use FFmpeg:

ffmpeg -i input.avi -c:v copy -c:a libmp3lame -ac 1 -q:a 2 output.avi

The important option is -ac 1, which downmixes the signal to one channel. Note that this will re-encode the audio, so expect some quality loss.

To change the quality for MP3, choose a different value (from 0 to 9), where lower means better. 2 corresponds to around 95 kBit/s per channel IIRC.

In Ubuntu, an old version of FFmpeg is probably supplied in the packages. Don't do apt-get install ffmpeg and rather download a static build from the download page, or use Libav, which provides the avconv command. It's a fork of FFmpeg with similar functionality and usage and available in the Ubuntu packages.