I think Amarok is the best music player available. It organizes your music in a good way and the various plugins gives you a lot of useful informations about what you are hearing.
Unfortunately by updating my Slackware to KDE 4.8, using Alien Pastures packages, my Amarok doesn’t play Mp3 files anymore. Ok, I now Mp3 is not the best format for many reasons (audio rendering, licence, …), but the most music I have on my PC is in this format and converting it to something else is not a good idea. Anyway when I try to start a Mp3 Amarok notifies me that he is not able to play this format and suggest to take a look in the FAQs. But i had no luck there.
On my KDE box I need to use GStreamer as Phonom backend because it’s the only that works with my configuration where the monitor is connected through HDMI. I want to use mainly the monitor’s audio boxes, so I have to use HDMI as the main audio interface and apparently only GStreamer makes the job.
GStreamer uses a lot of plugins to play different audio files formats. This plugins are commonly grouped into several packages: base, good, bad and ugly. The first one are the absolutely necessary. The second one are good quality plugins and a good license. Bad plugins are not to good about quality or don’t have some feature. Finally the ugly group are plugins with a good quality, but a bad licence… like Mp3!
So, I think you have already understood, to play Mp3s with GStreamer you have to install the ugly plugins package. For Slackware you can retrive it on Slacky.eu.