I’ve just recently added a config value to set the unfocused volume in CL#2341022, so you’ll soon be able to add:
[Audio]
UnfocusedVolumeMultiplier=1.0
To your Project’s DefaultEngine.ini file to achieve the same effect without setting the volume multiplier constantly. You’ll know when you’ve got this update because I removed GVolumeMultiplier at the same time.