My Sonos Beam started sputtering, then stopped working with my LG Oled altogether. I tried resetting the LG and the Sonos Beam several times. I called tech support for Sonos, then LG, then Sonos again.  Third time’s the charm. 

What we discovered, through the process of elimination, the problem was the Fire TV Cube. Not cutting it off after use led to a short circuit the whole eARC function. We disconnected the Fire TV Cube, then did a hard reset. We also did a hard reset on the LG and the Sonos, then everything worked perfectly.  For audio, on the Cube, I now use “Best sound Available” and not, “Dolby.”  My Sonos gives me Dolby for the Cube and everything is working properly.

If your eARC goes out. Try resetting your Fire TV or Apple, then reset Sonos and TV. Everything should work.