Ok
Maybe is something in your Settings, you are on Expert or Advanced in Settings?
When you first boot CoreELEC Audio default is analog pcm…
So first thing is change that.
Here are 2 screenshot from my HK1 RBOX S905X4 latest nightly NE 5.4.210, typical Settings for Audio
Then select receiver/many user check all
That should allow pass-through to Device TV/AVR/Soundbar etc…