Q: How do I get the keyboard volume buttons on my Mac keyboard work?

I am using a iMac 21 inch (2010) and recently the keyboard volume buttons have stopped working, maybe due to a Maverick update, or another reason...not sure but now when I try using the volume up, down or mute buttons on the keyboard it reverts to the desktop screen, regardless of which app I'm using. I've tried the system controls but no success. How do I reinstate my volume buttons?