I've not heard of this, and I'm flying blind here as I'm at work and can't test this...
But, could .wav files have somehow become associated with Media Player 10?
Does your PC play the 'Windows jingle' when it starts? If so, is it before or after Media Player 10 loads?
I find it very unlikely it is this, but might be worth checking.
Also, have you checked Start -> Programs -> Startup to see if there is anything in there that might be triggering Media Player?
Also, have you checked the Microsoft Knowledge Base on their website?