Try this:
Disconnect everything from the TV including the Arc and unplug the TV and Arc from power for a couple of minutes. Plug everything back in but keep everything disconnected. Be sure Anynet+ is enabled in the TV settings and set Digital Output Audio Format to Pass-Through and HDMI-eARC Mode to OFF. Run TV Setup in the Sonos app and follow the instructions. When the app instructs you, connect the Arc to the TV using the Sonos-supplied HDMI cable. After the Arc has been successfully connected, connect any other devices to the TV.
Keep the HDMI-eARC Mode setting to OFF when using streaming apps. Set it to AUTO when using the Blu-ray player.