Further developments:

On May 24th, a friend who used to be a service person for IBM came over. (She is now a top programmer for Sprint.) Apparently she gave off vibes that affected my computer, because as I was showing her how the MIDI files would produce sound but the keyboard would not, the keyboard suddenly started making sound.

Nothing had changed in the settings (MIDI or Record tabs) or the volume, which has always looked okay.

I noted the date and time when the sound came back so I can use System Restore if the keyboard sound disappears as mysteriously as it came.

Seriously, I think an update or a restart repaired or connected something, perhaps the wavetable that JohnInKansas referred to. Although after all the struggling I have done with this problem, I am not going to discount my friend's aura, either. It's just too coincidental that the computer suddenly started co-operating the minute she stood before it.

She says there's a program called Snag-it that I should have. Now I'm off to look into that.