I don't think I had selective mutism as a child and I guess I still don't but I do experience episodes where I "go mute". I've got social anxiety disorder (I've had it since childhood) and it can get pretty severe at times. It's always pretty bad but some situations are worse than others. In certain social situations (often when meeting new people for example) I freeze up (even if I know everyone in the room except one person), get all tense and extremely anxious and it's like I can't open my mouth and speak. I don't know if I get so tense that I physically can't open my mouth or if it's "just" all in my head. Anyway, instead of speaking I just sit there and panic on the inside while thinking that even if I'd like to say something I simply can't.

I know it might not be the same as what you're going through but at least it's similar so I get where you're coming from.