You need codecs. Amarok (the newest version) doesn't handle gstreamer backend at the time, so you can't use it, but instead you can use Xine engine and add mp3 playing support to it, thus making Amarok able to play mp3 files. Xmms does too, if you get the mp3 plugin for it.
For amarok: install xine engine (libxine and libxine-extracodecs [for mp3 etc. playback]), but not the Xine player itself (unless you specifically want to). You only need the engine, not the user interface. After this go to Amarok's settings and select Xine as the engine it uses; after this you should be able to play mp3 files.
In my own opinion gstreamer is better than Xine, but as Amarok is unable to use gstreamer right now, you'll want Xine instead. Or, for xmms, simply get the xmms mp3 plugin and install it, then enable it in Xmms settings.