I can only speak from my own experience, and since Helen Keller and I have the same type of hearing loss, no, there’s no “hearing” occurring at all.
To be able to actually hear, the sounds need to travel down the auditory nerve to the aural center in the brain. When someone loses their hearing due to high fever, the hairs in the cochlea, and possibly parts of the auditory nerve are damaged, so sound can not travel down the nerve at all, thus, no hearing is processed at all.
When I speak, I can feel the vibrations of sound around my head and in my neck and can distinguish the differences between SOME sounds—i.e. If I pay attention, I can tell if I’m speaking in a high or low pitch or whether I’m loud or soft. Some sounds vibrate more than others—but there is no distinction between the vibrations among the vowels, and many voiced consonants ‘feel’ the same. To make that distinction, I pay attention to mouth and tongue placement more than the vibrations.
I took speech training for 12 years—it’s mostly memorization, constant repetition, imitation through feeling vibrations by hand and never 100% accurate. It’s almost like a “Paint By Numbers” endeavor—you can memorize the codes, placements, mechanics, but it will never look/sound like the original. I speak quite well considering that I am thoroughly, 100% Deaf (which is rare, actually), and speaking is completely a cognitive effort, not a natural one.
From what I understand, from being exposed to information about Helen Keller my entire life, is that she learned speech the same way—memorization, repetition, cognition, mechanics, placement, etc, etc.