Keyboard Volume Controls Stopped Working...

The title pretty much sums up the problem. The volume controls stopped working a couple of days ago. The volume control up on the menu bar still works, as does the one in the Sound panel in System Preferences. when I press the buttons, the meter comes up on the screen and the little boxes turn white and it makes the little "splat" noise as usual (which increases in volume as usual??), but the volume of iTunes (or any of my other sound apps) doesn't change.

I've looked for some setting that might have changed, but I don't have clue...

Thanks in advance!

Dave