After doing more searching on this forum I found one posting where someone suggested increasing the mixing latency setting, which is a slider in the dialog found by choosing Edit>Preferences>Audio from the menu. He complained that after increasing the latency to the max, he got most of the missing notes to play, but there were still some that didn't sound during playback. I duplicated his result with my project.
I then tried decreasing the latency setting to the minimum, which worked for me. Why an audio setting would affect the BASIC PLAYBACK of an ALL MIDI file with NO AUDIO CLIPS is beyond me. Not very intuitive, IMHO.