bluetooth keyboard speaker keys not working

My old Bluetooth keyboard stopped pairing with my iMac so I took it to the Apple Store and they swapped it for a new one. It paired fine, but the function keys that I use to control the speaker volume up and down are doing other things, like clearing the desktop, etc. I went to System Preferences to make sure the Function Key box was unchecked, and it was. Tried checking and unchecking but the behavior was the same either way. I guess trashing the System Preferences file would be next? If so, where is it? Still using last version of Snow Leopard, not Lion.