Try this - Close your mouth, start humming and then pinch your nose/nostrils shut. You will hear the humming 'internally' just fine but externally, to someone near you, it will be quieter because some of the noise was coming out of your nose 'f' holes. After a couple of seconds you will notice the humming coming to a stop as the pressure builds up in your sinus cavities and ears and air flow thru your vocal cords stops.
I imagine a double bass with plugged F holes would make some noise but I would think it would be much quieter. It might be less prone to feedback on a loud stage.
