Thanks a lot for your advice.
Just for fun, I tested also LE 9 and as expected it shows the same behavior.
My decision is to switch to a Raspi.
The Wetek Core has never been a good choice, if you want to run new LE versions very early. 
Posts by fertigp
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As soon as I update my Wetek Core from 8.2.5 to 8.95.3 the available audio devices change:
Going to settings/system/audio - audio ouput device
8.2.5 shows ALSA: (Default) AML-M8AUDIO S/PDIF
8.95.3 shows ALSA: (Default) AMl-M8AUDIO Analog
As a result there is now way to output 5.1 sound to my Onkyo AVR, which is connected via HDMI cable. Especially passthrough is not working.
I tested this issue also with a fresh installation of 8.2.5 without any addons or major changes in settings and then update to 8.95.3. It is still the same.
Attached you can find the logfiles from both libreelec versions.
Looking at the kodi.log file of 8.95.3 the audio device is specified as: "No passthrough capabilities"