I have a temporary workaround that works but is obnoxious and I lose atmos. Tv on, audio output to arc, earc off, results on no volume through Sonos arc. 
 

change audio out to internal speakers by turning on earc. Change audio output back to arc. Then turn earc off and there will be a slight delay and the arc will work. Issue: you have to do this obnoxious process every time you restart the tv. And atmos is not a available. 
 

And to tease you, it will work for 2 seconds upon restart and then go sadly silent.  It’s an LG programming issue. Their customer service sucks. They’ve failed to schedule a tech to come out and try to fix it (code) no idea how that is doing to work, and failed to do so multiple times and it’s on me to call them back and keep pestering them to honor their garbage warranty and crap code.