For anyone who may have been following my saga as posted on 11/18, I can relay good news. The problem turned out to be in the radio after all. I replaced the original with another RBP unit (salvage), and voila! - sound!

What makes this a shock is that I had the original radio bench-tested by a reputable audio shop, and they gave it a clean bill of health. I tried a different amp, traced and checked all the wiring, etc., and couldn't find anything wrong. Out of desperation, I acquired a second unit. I now not only have sound, but the radio communicates with the 6-disc changer again, and the steering wheel-mounted radio controls work again, too. Hope this info. may help someone else in the future.