I've build a few iterations and suddenly it is broken. What!
Checking with the log file showed an error on a audio file name.
open ~/Library/Application Support/Godot/app_userdata/my-game/logs/godot.log
... **ERROR**: Cannot load source code from file 'res://UI/Audio.gd'. ...
So ... moving code from project using UI in uppercase to on that has ui lowercase is a known MacOS problem.