Restarting sound service?
You can kill the CoreAudio process by opening Terminal and running sudo kill -9 `ps ax|grep 'coreaudio[a-z]' | awk '{print $1}'`. It will restart automatically after a couple seconds.

That fixes some problems my aging MBP has been having, where it sometimes fails to detect headphones or decides the speakers aren't connected. No guarantees it will work for every audio problem, but it's worth a shot.

Source: zakgreant on macosxhints forums.