Does it work without it? You may have to recompile ffmpeg yourself with proper support for xvid/libmp3lame to get it working properly. Otherwise you may end up with no sound or just sound. If you get image without sound, try using mpga as the audio codec in n8x0 profile. I'll check if there are better instructions for jaunty, I just took them from the wiki which seems outdated.

EDIT: Added an easy to follow guide for proper ffmpeg on Jaunty to install instructions.