I wrote this with the point of view of web applications. But speech based disabilities could definitely be grouped separately. It would qualify as a disability because you are not able to communicate like everyone else.
Although I can’t think of any specific AT for speech disabilities, if there’s an alternative to the speech based inputs (like entering through a keyboard), I think the problem of accessibility is solved (as it’s about giving equal opportunities to everyone).
But your point about using speech as an AT is another fresh perspective that didn’t come to my mind. Thanks for sharing your thoughts 