My son was diagnosed with selective mutism about 6 years ago. Up until second grade he spoke ONLY to myself, my husband, my sisters and my parents. He had lots of friends, and did well in school, but wouldn't speak a word to anyone outside of the family. At home he was a normal, active, outgoing kid, but at school or in public he communicated only by gestures. He also wouldn't talk to anybody on the phone, family included.
I first noticed there was a problem when he was about 2 or 3, but the pediatrician told me he was "just shy" and he would grow out of it. I didn't become really concerned until he started school. Once kindergarten started and he still wasn't talking I began looking for second, third and fourth opinions. I took him to several psychologists and nothing helped. I also kept beating myself up because I thought "something" horrible or traumatic that I didn't know about have happened to make him this way.
One night though, just by chance, I saw a segment on 20/20 about selective mutism and it was a light went off in my head. THAT is what my son had, I just knew it. Once I had a name for it, I did research on the internet and found a specialist in my area. We made an appointment, and met with the doctor. He suggested Paxil, which I was strongly against, but I eventually agreed as a last resort. Well it was like a miracle because literally within a few weeks my son was talking!
Long story short, he only ended up taking the Paxil for a few months and we slowly took him off of it. He is now a typical 7th grader with absolutely no trace of the SM. In fact, he's constantly on the phone now!
PM me if you ever need to talk, and don't ever let yourself feel guilty for what your daughter is going through. The doctor assured me it most likely has a genetic component and has nothing to do with trauma, upbringing, etc.
Best of luck to you and your daughter.
Jynohn