Hi

The Sonos app is not designed to control your TV audio directly. The Sonos app (when you are in the now playing room using the volume slider) controls volume by communicating wirelessly with the attached Sonos soundbar which adjusts its volume.

If you are hearing sound over your Arc and internal TV speakers you either have not within the TV menu set the audio to external speaker and/or you need to run the Arc setup again via the Sonos app.

You can also try rebooting the TV by removing the power cord for 30 seconds and then plugging it in again.

Another possibility is that there is something wrong within the TV’s firmware/software that needs an update/reset which may require contacting LG Tech support.

If none of the above corrects the issue then I suggest you run a diagnostic within 10 minutes of the occurrence, post the reference ID in your next post and call Sonos tech support to review the data.

Just a clarification. There is no signal sent from Sonos to the TV via optical. Optical is a one way format, the Sonos accepts the IR signal from your remote, and adjusts the volume accordingly. The TV’s actual volume isn’t adjusted. 
 

CEC does work across HDMI ARC or eARC, in which the Sonos device does send a signal back to your TV, and the TV adjusts the volume, which then sends a signal back to your Sonos, which makes the appropriate change in volume. 

@Airgetlam 

Thanks. Got my wires crossed as I’m always thinking CEC. Corrected my post as well.

And technically, I’m wrong, too. In the case of CEC, the Sonos doesn’t receive anything from the remote at all, the remote connects to the TV, it is the TV that sends appropriate information to the Sonos across CEC to adjust the volume.