LG G3 has multiple settings for sounds. The "Touch feedback & system" volume is the one that controls the loudness of the full battery beeping. Go to Settings/Sound/Volume and turn down the "Touch feedback & system". It also affects the volume of key clicks but not ringtones, notifications or music.
Since I have stopped Google Now updates, I am now wondering if there is a way to tell when it has a new update that, potentially, has a fix for OK Google on "any screen". Does anyone know how to see the update release dates?
Same as Otis2015 described and using the factory installed Google app so I can continue to have the capability. Using LG g3 AT&T but it is definitely a Google App problem. I urge readers to go into the Google app, select the menu and use the feedback to let them now. Maybe that will help.