The approach most convenient in our use case (maybe yours is different): do not have any global sound, only add sound in selected scenes.
So at Settings, Audio, Global Audio Devices: make notes of te current settings if needed and then have everything Disabled.
At the scenes that need sound: add Audio Input Capture sources for microphones, USB mixers etc. Add Audio Output Capture sources to catch any (game) sound that goes to your speakers.
If needed, at those Audio Output Capture sources, go to the Audio Mixer's properties icon (cog wheel), Advanced Audio Properties and select the desired Audio Monitoring. (Monitoring is the sound not intended for the audience, but for the presenters and technicians. It is turned off by default since many people have their speakers close to the microphone.)
