Happens to me too. Volume cuts by about 50%. Only while running DCS, not in anything else. As soon as I quit the 'game' and go back to the UI volume returns to normal. Audio devices are left to default and all volume settings are at 100%.
Everything is affected, game sounds, SRS, discord but like I said, only with DCS running and in the game flying. It actually went away for sometime but recently returned, cannot say if it is patch related though. Very frustrating to say the last.