The audio output is set in Paper UI/Configuration/System to
Default sink = System speaker
and is working well.
As soon as there is a monitor connected to any HDMI-input while (re-)booting the device, the audio output is automatically routed to HDMI and nothing comes out of the audio jack.
The effect is reversible - when you reboot w/o a monitor connected, the audio jack is working again.
Can be quite irritating as one spends a lot of time with checking the audio configuration…
IMHO this is a desired feature of the used OS ( raspbian) and caused by openHAB.
Reading Here you will find this:
If your display claims to have speakers, sound is output via HDMI by default; if not, it is output via the headphone jack. This may not be the desired output setup, or the auto-detection is inaccurate, in which case you can manually switch the output.