So after upgrading to 12.04 on Ubuntu from 10.04, I cannot export my videos from kdenlive with MPEG-4 option, with the error message "Unsupported audio codec: libmp3lame"
After installing the lame library with the command:
sudo apt-get install libmp3lame0 (the last letter is zero)
The issue is still there, so tried another way next
sudo apt-get install libavcodec-extra-53
Now it works!