On the Spectre Blu-ray, the main audio track is a lossless DTS-HD Master Audio 7.1 track whereas the other audio tracks are lossy Dolby Digital 5.1. More than likely, your older TV can only pass through lossy Dolby Digital 5.1 because it is only equipped with HDMI ARC. Lossless audio like DTS-HD Master Audio and Dolby TrueHD/Dolby Atmos requires a TV with HDMI eARC.
If your TV had DTS pass through, you could get the core DTS Digital Surround track from a DTS-HD Master Audio or DTS:X lossless track, but it appears your TV doesn’t have this capability.
Check out this website for detailed audio information on all Blu-ray releases:
https://www.blu-ray.com/movies/Spectre-Blu-ray/281309/
If you are interested in getting lossless audio from your Blu-rays, consider getting an HDFury Arcana eARC adapter. It allows your Blu-ray player to bypass your TV and send audio directly to the Arc. I use the Arcana with my older TV and it works great:
https://hdfury.com/product/4k-arcana-18gbps/
NOTE: Sonos still doesn’t support lossless DTS, but (with the Arcana) you could set the Blu-ray player to convert DTS to PCM so the Arc will play lossless Multichannel PCM audio instead of DTS which still sounds great and is much higher quality than lossless DTS Digital Surround or Dolby Digital.