Success! Well, since I hate to rush into anything--and because I have two other computers--I put off this problem until only four days remained on the warranty. The Dell phone tech did a bunch of tests, and determined that my original motherboard was faulty. The Dell tech just replaced the motherboard, and the sound now works. There was nothing I could have done myself, as I had already updated the driver and done all the tests I could have. So it was hardware after all. Thanks to Chris M for all your hard work and expertise.