This is an utterly bizare problem I can reproduce 100% of the time.
My sound hardware is set to Analgoue Stereo Duplex.
Whenever I lock my screen (using WIN+L) and unlock, the hardware settings change to "Digital Stereo (HDMI) Outful - Analogue Stero Input"!
Which means I have to go to the Sound Settings to reset it.
I'm running 11.10 on a MacBook Pro. Everything else seems to be working normally.
Any idea what's causing it and / or how I can stop it?