Sorry, I've been a member forever, but I rarely post, and don't know what a PM is. However, I do have some information for you. A woman named Torey Hayden wrote several books about her teaching experiences with special needs kids and her speciality was elective mutism. Not sure about ant hard facts more about how she reached these kids. The books were written in the late '70s and early '80, so they might be out of print.
Lisa