Audio doesn't care about monitors. You can't tell OBS to capture audio only from one audio, because Windows does not associate audio with monitors.
You're probably capturing your default audio device, which means you're capturing everything Windows produces.
If you're using Voicemeeter or Virtual Audio Cables, you may have one or more virtual audio devices, so you can capture specific applications, or avoid capturing audio from certain applications (skype, other chat apps, etc).
If changing the active application (in order to interact with OBS) affects your audio, that's likely a function of how the app you are capturing works, or because Windows is set to change audio settings based on some other preference.
This might help track down what it is:
https://www.windowscentral.com/how-manage-sound-settings-windows-10-april-2018-update