This might have to do, that mp3 is not a free audio format and is by default not support by linux distros. You have to install restricted plugins or just convert the mp3 file to ogg. I would suggest the later. For conversion use "Soundkonverter" or if you are familiar with the commandline: ffmpeg -i file.mp3 file.ogg Then try again. It might help.
Thanks for you answer, I forgot to mention than I already have *ubuntu-restricted-extras installed and still doesn't work. Using ogg (and wav) files works.
Yeah...I think in the end it's a legal issue regarding software using mp3 (proprietary format). My weapon of choice: install this service for dolphin (if you are using KDE) Convert to mp3/ogg within dolphin. It let's you quickly convert audio/video to ogg via the context menu of dolphin. cheers