Nice avatar haha

You know I honestly do not know why this would happen to you. But next time you get lucid, just take a few moments to calm down and just maybe remember some times where you have spoken in real life. Remember how it feels like and what your voice sounds like. Once you have a clear memory of this, try speaking. It should help you speak regularly. Plus having limited abilities in dreams is a normal thing. You have probably had experiences like these in nightmares.

Hope this helps!