I have a LOT of problems with switching jacks in some of my game boys. It sounds silly, but try plugging/unplugging your headphones a few times. That usually fixes it on my Light (but that one's pretty stubborn so it comes back).
Outside of that, it's probably a connection issue, and then I'm as lost as you are right now. But, you know, sometimes it's as simple as a big piece of something getting itself into the switching jack.