Sorry, haven't much time so I am just going to copy and paste from an email I wrote to someone:
My daughter is now 7 and has completely grown out of her selective mutism. She can still be a bit shy, but she will mostly reply - even to a stranger - when asked a direct question. I think she did just grow out of it.
Her selective mutism started when she was three and going to nursery. She didn't speak at nursery at all, not even to a friend she would speak to outside nursery.
At her worst (somewhere betwen age 3.5 and 4) she would only speak to me, her brother and two friends. She wouldn't even speak to her dad for a bit because he had been working away!
She started school aged 5 (normal in Scotland) and didn't speak for the first 6 months of school. She started by whispering to the teacher, then whispering in small groups. Now she has no reservations about speaking in class at all, and had loads of friends.
I was delighted recently when the teacher described her as "a chatterbox". She also took part in the nativity and had to say a couple of lines in front of the whole school and audience!
I don't know what finally made her decide to speak. Her teacher was really patient, but at the same time made it clear that she would really like to hear what dd had to say.
They also did some sign language in class (the teaching assistant knew sign language) which helped her fit in a bit. She also had a few picture cards for urgent needs she might need to communicate (such as toilet, or not feeling well)
One tip that I was given was if people were saying she was rude or shy was to say "No, she isn't rude/shy, she will speak when she is ready" This means that you are letting people know without having to explain in front of her, or having her labelled as rude or shy