And there's more - looking at the sound preferences on OSX using the "hidden" menu ...

Hot on the back of my previous two posts: -

Mac OSX - Checking your WiFi signal strength

More hidden secrets in OSX - battery strength indicator ...

I've just "discovered" another one.

If you hold down the [Alt] key and click on the sound/volume icon   you get this: -

which is very very useful, especially to me with an external Plantronics CS-60 USB/DECT headset.