I think the problem may be in your TV, which is supposed to be separating out the audio signal, and sending it to the Sonos.
The first thing I’d do is remove the optical connection to your Amp. Leave the Sonos Arc connected to the eARC port on your LG. Of course the Blu-ray player will stay connected to a separate port on the LG.
Check the software on the LG’s OS for updates, and apply them as needed. Then unplug the LG from the wall for two minutes. Do not just turn off the TV using the remote or the power button on it, that merely puts it to ‘sleep’, and does not force a reboot of the OS.
Plug back in the TV, wait a couple of minutes for everything to load and reconnect. It might be worth, at this moment, reseating the Sonos supplied cable between the Arc and the LG, just to be sure everything is making appropriate contact in the HDMI connections at both ends. Then test.
If this doesn’t work, I might submit a diagnostic to Sonos and call them, but it’d be much more likely to call LG’s customer support.
