Everything worked fine (sound enabled with mp3 codec selected and video playback) after I rebuilt ffmpeg. I'll post the link I had to follow, which wasn't the one in the Ubuntu wiki. That one came to be some way outdated.

So I have it working, and in the next days I'll be testing it deeply

LINK: Install FFmpeg and x264 on Ubuntu Hardy Heron 8.04 LTS, valid also for Ubuntu Jaunty

EDIT: If someone happens to read this, I ended up switching to Debian Testing, where everything worked fine with the Debian Multimedia repositories enabled. Although things worked in Ubuntu, regarding to VLC, FFmpeg and Knots, I experienced some terrible performance issues with the Atheros wifi driver.