I think it's cause of the pressure you make to talk when there's not enough air or it is caused by the spasms in your mouth muscles. I had this problem for a long time and now that I continued my therapy (after 3 years of giving up), I can see it was all because of pressure and spasms my face and body had.in my case, I got a lot of headaches and pain in the chest. now it's all better now just by doing breathing practices and lecturing for myself in the mirror. feel free to ask any question. we all are here to help each other :) one tip that helped me a lot was this: "Control the air with your diaphragm, not your chest or neck". every time I lose focus, my therapist tells me this and I get control over my body again!