I have an LG UBK90 Blu ray player and an LG CX OLED TV with a Sonos Arc connected to the eARC output of the TV. I have the Blu ray player set to output multi-channel pcm and the TV set to passthrough. Like GDB, when I start the TV and player, the Sonos only sees stereo. But if you change the HDMI input setting (from bitstream to ppm or vice versa -  it doesn’t matter which!) the change seems to make the Sonos awake up and read the 7.1 multichannel. It then stays that way until you turn the tv and the player off. Then it forgets…….

Very frustrating. I usually get around it by using Plex. I have all my Blurays and DVDs ripped there and I use a MacMini transcoding to avoid DTS going to the TV. 

I should have added, that the TV is set to passthrough, so the fact that the original encoding was DTS should be a complete non-issue. The Blu Ray player decodes the disk to multichannel PCM. This then passes through the TV and the Arc should recognize the multichannel PCM, BUT IT DOESN”T! This must be something to do with either the way the LG player codes the PCM signal or an issue with the Arc’s ability to decode it….

Or the TV should tell the Arc it wants to send multichannel PCM and doesn’t (in the right way or at the right time). This could also be a TV problem.