Okay, I'm really dumb. It was the first thing I should have checked. I still had the TV antenna hooked up to the TV. I thought I had disconnected it ages ago since I watch TV through WMC on my computer these days. Or I will again after I update my Nvidia drivers, anyway. In any case, I noticed it was still connected. I unplugged it, and now it's eerie silence when it should be. Except for the sound of the world's longest duuuuuuuuh.