I have some speech difficulties, but I am not sure I could classify them as mutism. My voice will falter, I sometimes stumble over words, but it is not really a stutter either. A noise that is not what I planned comes out, then I correct it, sometimes mid-sentence. Sometimes I have to stop and then restart. Does that make any sense? (So I answered "Other") above.
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However, when I was very young, until I was nearly 4 years old, I did not speak intelligibly at all, only stringing out long "sentences" of jibberish. This is said to have been due to my dyslexia, though I think it also had to do with being threatened with death, should I tell about what was being done to me. That is just a guess.
Added in another edit:
I just remembered, I did have it one time when we saw a program on TV about sexual molestation of children. I could not speak to tell my husband what had happened to me. I did cry though. Finally I wrote it down. I could do that!