I have an alter or many alters like this. Who are mute. Some of them can write. Some of them have nothing to say at all, except to me. My rageful alter is mute, just like you described. He sure talks a lot to the system, but he doesn't speak outside of him. I actually am starting to learn that many of my alters don't speak, but love to write. I have maybe a handful who do all the speaking. Some of them will only speak in front of certain trustworthy people.

It has happened, but few and far inbetween where a mute alter is present and will not talk to people. Can't talk to people and nobody takes over for her. It happens a lot in more stressful situations and it happened in therapy once. I wasted the entire session not talking, begging myself to, but not doing it. No matter what she did to try to coax me to talk I couldn't do it. I shouted and shouted at her "speak! what are you doing!? SPEAK".