[PROBLEM] Losing sound with pulseaudio

[Nachteule]

Aha, but I still think you have a general hardware problem.
As a test I switched to Pulseaudio on my Pi3+ for a while. But do not think that leads to problems here

Something else about passthrough:
If you enable this on PI:HDMI, it will only affect this device. So if you switch to Pulseaudio, this setting is completely irrelevant.

This must be activated differently, see for example https://kodi.wiki/view/PulseAudio
how this should work without running X desktop I can't tell you. But it doesn't matter, as long as Pulseaudio doesn't run stable in general it's a wasted time to deal with passthrough further