From Amarok Wiki
If you're running Amarok on FreeBSD 5.x or 6.x, you can make it play MP3 by installing the package multimedia/gstreamer-plugins-ugly from the ports, besides Amarok's own dependencies. Installing this package (found at http://gstreamer.freedesktop.org/; look for the gst-plugins-ugly package) should also help for other Linux distributions or other operating systems.