I'm pretty sure I have selective mutism. When I'm very stressed, anxious, overloaded, or experiencing a strong emotion of any kind, I can't get my vocal cords to cooperate no matter how hard I try. Sometimes I can say a few words, but not loudly/clearly enough to be understood, and sometimes I can't speak at all. Also sometimes my brain will decide it can't do words anymore, sometimes suddenly, even when I'm mid-sentence. When I can't speak understandably/at all, I'll try to communicate by pointing or using gestures, and sometimes using vocalizations of some sort, which I tend to be capable of even when I can't form words. When talking with people I don't know well, trying to initiate any kind of social interaction, or when I'm not sure exactly what to say, I usually am stressed/anxious enough that I can't speak. I have a text-to-speech app on my phone, but my parents don't let me use it. They insist that I just don't want to talk and try to force me to do it. No one I've met in person believes me when I try to tell them that sometimes I literally can't speak understandably/at all. I'd love to be able to stop talking altogether, but my parents would have none of that. Communicating in writing is much easier for me.
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Yet in my new wildness and freedom I almost welcome the bitterness of alienage. For although nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men.
-H. P. Lovecraft, "The Outsider"