We know what part of the brain is responsible for speech (Broca's Area) and that damage in that area can cause speech impairment (even Sign Language) but yeah, thats pretty much all we know in that regard. ​ I also assume that when the sympathetic nervous system is activated stutter can also occur due to muscles tightening up, including the jaw muscle and vocal chord, but what we do fail to understand is why some people sympathetic system acts up with no good reason, what underlying and unconscious processes occur, maybe even trauma, but recent studies show there is a high chance of reversing that with psychedelics.