What an amazing article!!!  (Link below)  If you are going through the diagnosis process or are searching for what may be causing your child's abnormal behavior, please read this article.  When we started out on this crazy journey, so many people tried to tell me what was "wrong" with my son.  We heard everything from "he's just shy," to apraxia, to autism.  Our original pediatrician told us it was probably autism and I wanted to scream "Are you crazy!!!"  As a parent trust your instincts!!  You know your child better than anyone and in my opinion, most doctors have none to very little experience dealing with SM and SPD.  I knew that my child was extremely loving, cuddly, tuned into emotions, and verbal in our home and I am so glad I did not accept people's opinions as fact. I think the word "autism" is thrown around so loosely now and people just accept the diagnosis.  Granted for several it is the correct diagnosis,  but selective mutism is NOT autism!
It is hard work to find the correct people who truly understand the treatment your child needs, but don't give up.  Feel free to contact me to learn more about where our child got treatment. 

The Most Common Misdiagnoses in Children

http://childmind.org/article/the-most-common-misdiagnoses-in-children/