Sorry, @chscag I was aware that the article came after you wrote to this thread, and didn't mean to imply that you'd missed the memo.
I resolved my own issue with QTP7 not playing audio files. I had been using it to play copies of files from my iTunes library — either m4p's or m4a's. Some of those I had saved as mov's. It so happened that the ones I tested were either m4p's or mov's exported from an m4p. The difference between protected and unprotected didn't matter to the app when it was running under Sierra, but now, under High Sierra, it made a difference.
Odd, but resolved.