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.