@Hubert-Thoring @Dave-J-0 @Peter-Schiefer
Feedback from another hour of testing:
I tried all possible combinations: first music on, then CarPlay connected, then sometimes music off, sometimes not, sometimes CarPlay first, then music etc. The result: with the new iPhone/iOS, all sounds (music or turn-by-turn announcements) are played exclusively via the motorcycle speakers. With the old 6S, no sounds are possible in this way, neither on the motorcycle nor on the cell phone.
And now I have decided not to invest any more time. I'll just use the app, which looks really super informative on the motorcycle display, without sound.
Thanks again to everyone who thought along and tried to help with tips!
Wolfram