I've had the volume and disk eject keys "break" every once in a while on my Cube, usually after 2 or 3 days of continuous uptime, and rebuilding the desktop usually straightens it out.
I have discovered that there's an extension that controls these buttons, although it doesn't say so in the description in Extensions Mgr. If you've indescriminantly turned off any extensions that it didn't seem like you needed, (sorry, I forgot which one it was) try turning them back on.
CV