Internal Audio hardware disappeared!
I tried to configure my USB phone to work with Skype without success, although I could get the phone's mic and speaker to work using Sound Preferences.
I decided to give up on the USB phone and so reset "Internal Audio" as the preferred device. All seemed fine until a minute or so later...
I was listening to music when all went silent. After some investigation, I see that the "Internal Audio" device is now missing from SoundPreferences\Hardware and the Audio panel icon is displayed with an "X"
The USB phone is recognised as an audio device if I plug it in.
The PC sound works fine if I boot to Windows, so the hardware is OK.
Re-booting makes no difference.