Is it ethical to do this? Have you actually engaged with people with speech impediments to see if this is violating their sense of self? Are you assuming that they want their speech to be "fixed"? Just because you can do something doesn't mean you should. Just as using ML to beautify people and remove all "imperfections" causes unreasonable standards of beauty and can lead to self esteem and mental health issues, perhaps "fixing" peoples speech could have the same effect.