Thank you Martin!
I continue wonering on that miracle.
Now I discovered that if I remove all other tracks and keep only the audio track - the sound moves back to the external sound system (vie the headphone jack).
I meen - the headpone jack is pluged in all the time.
SO, when i remove everything but the audio track the sound goes to the external sound system via te headphone jack.
Even if I add only one midi track (even empty) the sound moves into the laptops loudspeaker.
If I add an instrument track instead of midi track, then the sound doesn’t move to the internat loudspeaker, but stays as it should connected to the external sound system.
So obviously the appearance of even one miditrack moves the signal from the headpone jack to the internal speaker of the laptop.