OK, do (in KDE):
A box opens. Type *.mp3 into the search field. Another thing opens below it, called Audio. Click it, you see MP3, click that. The right pane opens. Filename patterns are *.mp3, *.MP3. Further down I get a choice of kaboodle, noatun, Xine and xmms. Click whichever one you want, then move it to the top. If your system cannot find the first application in the list, it tries the next etc. Neat. Easy. No files to edit.