The coordination between all the parts of the “speech machine” is the problem. When humming your are using your diaphragm and vocal folds, When speaking you are now ALSO using all your articulators: jaw, tongue, soft palate, lips Many times the problem is with the muscle coordination, NOT the vocal folds. The majority of the time when a person is stuttering the vocal folds are in the correct position for the intended phonation and not slammed shut like many people think.