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Current versions
xine-lib (>xine-lib-1.1.2) always includes MP3 support through ffmpeg dependency.
Older versions
Amarok may say that the xine backend (xine-lib-1.1.2) does not support mp3 playback.
After confirming that the USE flag list for xine-lib contains mad or mp3 you may want to try renaming (or removing) your ~/.xine directory[1].
[1] http://linuxfud.wordpress.com/2006/12/03/amarok-suddenly-stops-playing-mp3s/