Hi Deano,
many thanks, mate. Giving your links a read now.
Switching between Tele (HDMI) and main Speaker at the PC’s desk works fine again and immediate. Awesome. I hope, it will stay that way.
Lesson learned for a Newbie like me:
If some applications demand Pipewire and some other applications want Pulseaudio, it is INCORRECT to install both such packages by same name.
Instead - as you explained - the combo has to be:
1. pipewire
2. pipewire-pulseaudio
3. pipewire-libjack-0_3