You aren’t going to believe this, but I did the mixdown of the audio, re-exported to QT, and the DVD burned perfectly (!?!)
[I also trashed the plist file, but I had done that before and it didn’t seem to help the first time, so I assume it was the audio mixdown!]
So, thank you so much to all of you who helped, especially to “Thax” – you saved my little butt.
Yours in gratitude,
Wayne