clipped from: www.interwebworld.co.uk   

1. You need to add the Multiverse repository to /etc/apt/sources.list. Add the line deb http://nl.archive.ubuntu.com/ubuntu/ dapper multiverse. Apparently you modify the nl part to the country you’re in, so mine was gb
2. I think you have to do a quick update here, so from a console window run sudo aptitude update.
3. When that’s done, run the command sudo aptitude install libxine-extracodecs. This will install your mp3 support.
4. Run amaroK. You should probably restart the sound server with killall artsd, or reboot, but I found the mp3 support took effect straight away.