Mainstage change volume level of external programs

Hello community! I recently started to use MainStage 3 for my live gigs and practice. And I noticed strange behaviour - when I use Mainstage simultaniously with another sound producing application, for example run accompaniment from youtube in browser - after some time volume level from youtube changes, usually becomes much more quieter. If I pause and resume yoputube playback volume level restores to normal. And if I exit MainStage - youtube level returns to normal. How can I disable such behaviour from MainStage side?