Ok, this is an embarrassing problem. I'm using AVR Studio 6.1 beta (6.1.2440), and about every 5-10 times I build my solution, the Benny Hill chase music starts playing!
I have visual studio 2010, and it doesn't do it.
I've googled for viruses, but found nothing. Windows defender found nothing. (My box is pretty well locked down - I don't run as administrator).
It seemed to start happening after I installed the LUFA via the gallery - this may be a coincidence.
Have I found an easter egg? Can it be turned off?