muting/unmuting audio on screen lock/unlock in gnome

I constantly listen to an online radio at work. Not because I like the music, but because it's a handy way to detect network problems.

Whenever I leave my desk, I lock my screen. And when I come back later, the volume of my speakers is turned down most of the time. My colleague doesn't like the music either ;)
What I want to do, is mute the audio when my screen is locked (because I won't be there anyway), and unmute it when it's unlocked.

There seems to be no way to call a script when the screensaver is activated, but the gnome-screensaver sends an event to DBUS which I can catch.

More information on this in the Gnome FAQ. I need to get my hands on "dbus-monitor" :)