Hi,
Not going to be much help here as I have the same issue but with a CM4 on a CM4-POE-UPS-Base board. I used Pinn-Lite to install RaspiOS and LibreElec as dual boot.
With a Sony HDTV connected to HDMI0 I get audio when booted to RaspiOS but no audio and no HDMI select option in Settings/System/Audio/audio output. I only have Bluetooth default option listed.
I assume there is a config setting in LibreELEC that is not set correctly to see the HDMI port.
Hope someone can help with this.
New install on the above device. Install was using Pinn-lite (v3.8.7)with two selections:
Raspios_arm64_full
LibreELEC_RPI4
Installed with no issues expect No HDMI audio with LibreElec.
In Settings/system/audio/Audio Output Device there is only one option: Bluetooth Audio (PULSAUDIO)
So seem to have no way to select HDMI.
As this is dual boot, when booting to the RaspiOS I can play MP3 with no issue through HDMI audio.
With no changes except to reboot to LibreElec and then no audio.
Please help if you can. Thanks. 
