Issue with losing ability to change volume

I have a 06 iMac 24" and I have the latest released Lion version running. Every now and then I will come back to my computer and find that I can't change the volume at all. The speaker icon will be grey instead of black and when I try using the volume buttons on my keyboard I get the usual speaker shown on the screen but it then also shows a circle with a line through it below that. I have to restart the iMac to get audio controls back.

Any ideas on how to fix this? My iMac doesn't go into the normal sleep mode....the screen with go black after a set time and thats it. Only thing I usually have left running when I walk away from it are iTunes and Colloquy.