How do I make my “Fn + Volume Up/Down” keys adjust my headphone channel in Ubuntu Karmic?
you can use xbindkeys:
Run xbindkeys -k in a terminal. Press your keybinding, and add the appropriate line to your ~/.xkeybindkeysrc. for example, With an 'amixer' command to raise/lower your headphone volume.) Don't forget to have xbindkeys start before your windowmanager. You can have a look here for more on xbindkeys. This way, you're not dependent on DE, and you can even transfer you keyboard short cuts to other distros/releases.
(I know it's not exactly what you want, but it might solve your problems?)