Could be what Tallyman said. It has happened to me when using wireless headphones. There are also viruses/trojans I don't know the exact term, that play sounds during what seems like random timing. There used to be a virus/trojan out there that would play phrases by a southpark character named "Cartman" randomly in the background. You could also have a pop up playing in one of your browser tabs, who knows.
_________________
Don't you mind people grinnin' in your face