I've just got Rift S and have troubles with sound source switching when I put headset on. In some games it keeps coming from my desktop headphones. And when I open virtual desktop from oculus overlay (button on the right controller) the sound starts coming from headset speakers. I think that's only when "sound mirroring" from oculus program is working — when the overlay is opened. When I close it sound stops again. 
I mean I could just unplug headphones or disable them in windows before playing but that's a chore to do each time. 
What is the right sound source setup, what am I missing?