The four instruments in my first post, plus "Nice Piano," are inaudible. All other piano instruments work just fine. The path is correct. The speakers are on, both when the control handles are displayed and in the mixer.
This is baffling, and I have no idea what to try next.
I realized that the drive onto which the Gold base had been downloaded was having problems with bad sectors. After running some disk utilities, I removed Gold base from HA, then downloaded it from Myriad again, and reinstalled.