It actually can! Since the RF can see inside your mouth the system doesn't inherently need lip movements and can be fine tuned on what we call "internal speech". Note that this isn't the automatic tongue movements that occur when "thinking" words to yourself, but it's rather like mouthing with your lips closed. We don't often lead with this since it's weird for many people to do, but with a bit of practice, we have been able to get it down pretty well.

But how would you distinguish all the sounds that don’t require any tongue movement, like (most of?) the vowels?