Gee... I wish I knew the answer to this. I don't. I do a lot of this kind of muttering. But, as far as I know, I don't do it in my sleep. Perhaps other members, here on PC, will have some useful suggestions. The only thing that occurs to me is that perhaps talking (as with a therapist) about what causes you to feel the way you do might help to relieve some of the pressure that's driving your "sleep talk". According to WebMD, there is no known cure for sleep talking:

https://www.webmd.com/sleep-disorder...n-your-sleep#1