Mine does this too. I wouldn't worry about it. Macs don't have a BIOS and have to emulate it as a result. The time gets messed up too which you might notice upon boot before the boot camp manager corrects it (or OSX corrects it when rebooting into it). My previous Mac had some question marks in the device manager for things like the motion sensor (for the hd I think in case it is dropped) and Apple said it was normal.
My guess is the BIOS emulation on occasion tricks Windows into thinking there has been a hardware change with the nvidia audio. It could very well also be related to graphics switching too, but also because of the BIOS emulation. Again, I would just ignore it. It's just a little box that pops up and it's harmless. I installed a newer nvidia driver too with an inf I modified myself but it was doing this before then so that's not the cause.