Go to Settings, Sound, Manage Sound Devices, and Scroll down to your input devices, and disable all of them Except for the one you're wanting to use. I'm guessing the Game just picks one from the list without even thinking, and since there's no in game option to change it, really the only way to change which mic it uses is to disable the other ones. If you need those sound inputs, you can always scroll back down and Re-Enable them