Don't know for certain.
But seeing as you was downloading games,
I'd guess you were using kazaa or another crappy piece of P2P software.
THEY ARE FREQUENTED WITH WANT TO BE HACKER "SCRIPT KIDDIES", FILLED WITH "CRAPPY SOFTWARE" THAT TURN INTO VIRUS AND MANY MANY OTHER BAAAAAAAAAAD THINGS!
my advice to you is to check trend micro's free online virus scanner, do a search on google.com for trend micro housecall and you'll find it, its free, reliable and gets rid of virus's.
By the sound of things, someone's either fucked around with your onboard speaker or setup a virus to do that.

What a bunch of wan*ers.