I hate Ubuntu >:(

In Sound Preferences, if you turn the volume down to the bottom, it mutes automatically.  Ok.  OS X does this, too.

But then if you drag the volume slider back up again… it stays muted.

>:0

If you set a USB audio device as the default in OS X, unplug it, and replug it, it will become the default again.

If you set a USB audio device as the default (the “fallback”) in Ubuntu, unplug it, and replug it, your settings have been forgotten.