I had a realisation today which I am deeply embarrassed to have taken twenty five years to figure out. However, it's worth sharing.

I happened to walk past a hearing person using their voice (plus signs) with a deaf boy who admittedly likes you to do that. He replied using speech which, to be blunt but accurate, was not understandable. I realised that since there is nothing wrong with his mouth, tongue, lips, jaw, vocal chords etc, the parts of his speech that are missing or indistinct are missing or indistinct because he cannot hear either other people making them or himself trying to make them.

No shit, Sherlock. Well, indeed. As I say: feel a bit foolish.

I checked this hypothesis out and it turns out that deafened people's speech, people who were hearing and had perfect speech patterns, lose parts of their own speech in a pattern that exactly reflects their hearing loss (of course!)

Therefore, hearing people can get a VERY good idea of what their own speech sounds like to a particular deaf person, by paying attention to the voice of that deaf person, and that can be an enormous help in reminding you of the need, sometimes, to stop relying on it.