Jonaz, I've wondered the same thing...
I can't speak for your teachers, but mine told me when I asked that silently vibrating the mantra is much more powerful, even though there's nothing "inferior" about chanting aloud or saying japa or soft whispered repetition.
Hindus, I have heard, believe the same thing.
My own personal opinion is also that, when you can vibrate them internally without distraction (Even when there are other noises or people speaking) it leads to greater internalization of the mantra or Dharani. This will especially help, in my experience, with recalling the mantra in dreams or in liminal states of consciousness like just waking up. Or even at the time of death.