I’m trying to figure this out for my Panasonic UB-420 as well. Based on what I’d read (https://www.trustedreviews.com/how-to/get-dts-sound-on-sonos-arc-and-beam-4040661) I thought I’d just need to set the DTS audio output to “PCM” in the blu-ray player settings. However, the blu-ray player’s playback info and the Sonos app still show it as 2 CH / stereo PCM. This with a TCL 8-Series set to auto-passthrough. If I change thee TV settings to auto-detect, the Sonos app shows it as multichannel PCM 5.1, but the blu-ray player playback info still says it’s only 2 channel PCM. 

I have the TCL 6 series, which has recently been updated with the eARC upgrade. I am thinking that LPCM pass through isn’t supported because when I try uncompressed 5.1 on my Xbox Series X it says it’s incompatible.

Dang, that would be a bummer...I didn’t think that they could add eARC without supporting LPCM. Wirecutter says “However, we’ve confirmed with TCL that the update to Roku OS 9.4 this fall will add support for eARC, AirPlay 2, and HomeKit. The HDMI inputs support a bandwidth only up to 18 Gbps, though, so they can’t accept 4K signals at 120 Hz.” Could the bandwidth limitation also prevent LPCM?