I ran into the exact same issue people outlined in another thread now closed of a similar title. I managed to solve it for €30 and have so far not run into any issues.
I used a HDMI splitter where the EDID can be set to tell the UB820 to output multichannel PCM (as it seems the C9 tells it to output stereo). I plugged the UB820 output into the splitter input and then the output of the splitter to the TV (not into the Sonos ARC).
I have an LG C9 and Sonos ARC and I'm now getting Dolby atmos with atmos discs and 7.1 LPCM with DTS MA/HD (haven’t tested DTS X yet) discs. No issues with Dolby vision either. I set the splitter not to copy the EDID of the tv (as this is the issue) but set it to the Dolby Atmos/vision/7.1 EDID setting. UB820 set to PCM for DTS (secondary audio to off). C9 set to eARC and pass through. Great solution to the PCM 2.0 issue for 30 quid.
This is the splitter -> ROFAVEZCO HDMI 2.0
Splitter 4k 60hz 1080p 120hz (EDID 7.1 setting). Available on Amazon UK.
Maybe someone here will flag an issue I haven’t thought of but for now it’s working for me.
