All media players support mp3.
but some distros remove the support on pre-compiled binaries. (for fear of being sued)
there are 2 options....
1) use a idfferant format, OGG is much better than mp3 in my opinion.
2) uninstall kubuntu's versinon of Xine, and use un-crippled Xine from http://xine.sf.net
you can convert mp3 to OGG with programs like sox http://sox.sf.net
( might even be already installed on your system )
sox myMusic.mp3 myMusic.ogg
sox reads the file extension to work out what formats to encode to, and virtually all sound formats are supported.