NOW THAT IS SOMETHING I'VE heard of.
Over in the Samsung forum you hear the same complaint about sound that comes from the optical and stops when Netflix plays.
Look around the Netflix menus for any control for the audio and again see if there is a setting to stereo or PCM and such.
But be sure to get LG to say this Netflix works with the optical since this is a complaint in other Smart TVs.
And finally, LG may not consider it to be their problem as Netflix wrote the app.