Hi, Libby,
I was just reading a recent issue of Discovery magazine and it featured a young girl with selective mutism. What they determined was that it is a form of social phobia and can be treated with therapy. Maybe it's something you might consider.
best wishes, Sue
my son is now 8,, when he was in nursery they approached me as he never spoke a word would play get along with anyone thing is you couldnt shut him up at home , he went to speech therapist as his speech was slightly delayed more cause he couldnt seem to get the quieter letters,, anyway the nursery raised there concerns with the speech therapist and he got diagnosed with selective mutism,, he is alot better today but if he is told off he still wont respond,,, your little one may have the same problem