PROBLEM SOLVED.
I downloaded RealTek AC97 audio driver package a3.97, ran it, restarted the computer, went to Device Manager and checked the settings to make sure all sound devices were using this driver. Then I went to "Sounds and Audio Device Properties" and selected, menu by menu, the new RealTek driver for all devices.
Everything seems to be working fine now. These low-end Yamaha desktop speakers even sound better than ever. Literally, stereo quality.
I'm a happy man.
I hope this helps someone as much as "Bookworm"'s myriad advices to others have helped me.