Which version of Slackware are you using? Did you update any files?
What music format are the songs in? ogg? mp3? m4a?
It sounds like you may be missing necessary libraries. You can open a terminal, and run the command xmms, and you may see an error about a missing library, then you can install that missing file.

if you want to remove files in Slackware, use the pkgtool command as root.