Lots of musicians mute their speakers when they're not recording. After all, do you really want to hear someone's MySpace music on thousand dollar speakers?

They also tend to mute all application sounds, as it's really annoying to be paying attention to some very delicate section in a track only to have an alert sound blow your ears out.

I also keep my speakers muted during reboots to avoid that Mac bootup sound. It tends to also be painful through said large speakers.