Buy a 2019 Shield Pro, not the tube model, and connect it to a HDMI input on the B9.
Shield ← HDMI → LG ← HDMI-eARC → Sonos
Make sure the LG & Sonos are communicating via HDMI-eARC, not HDMI-ARC.
The B9 and the Sonos ARC support HDMI-eARC. Reference: LG, Sonos
The Sonos ARC supports TrueHD + Atmos. Reference: Sonos
The Plex app on the LG cannot pass TrueHD audio via HDMI-eARC to attached audio equipment due to LG restrictions.
Devices attached to the TV’s HDMI inputs can pass TrueHD audio via HDMI-eARC to attached audio equipment.
Do not purchase the non-Pro Shield. The Plex app on that model has issues with high bit rate video and audio such as 4K HDR movies with TrueHD audio. Search the forum with “Shield Tube” or similar terms to see the posts.
See this thread for additional details:
