So to continue .... since it was given to us by fruitylips, we've had our LaserDisc player hooked up on my receiver's VCR2 input because, well, at the time my satellite receiver was hooked up to the LD/SAT input. Yesterday while troubleshooting, I noticed that the LD player -- for which I lack a manual -- has an AC3 RF audio output. So today, I disconnected the satellite receiver (which we're not using anyway), hooked up the LD player to the LD/SAT inputs, and did some checking . . . . . . . . hmmmmmm. Turns out I can get audio from the LD player in analog component-audio mode, but not a damn thing over the AC3 RF connection.
Since it seems a far stretch of coincidence for both the LD player and the DVD player to have independently developed AC3 output failures, I would say this is strong circumstantial evidence to narrow the diagnosis down to a failed AC3 decoder in the receiver. Which, of course, would probably be the most expensive of the three to replace.
Bah.
I don't know whether it's repairable or not. I suppose I'll have to find out.
(For those who care, the receiver is a Pioneer VSX-906S, the DVD player is a Pioneer DV-606D, and the LD player is a Pioneer CLD-D504. Yes, I like Pioneer A/V equipment ... it sounds great, and Just Works. Except now.)