My DD (age 12, PG) was DX with Selective Mutism when she was 5. It confirmed what we knew from 18 months of age. I don't recall her ever talking in public. She was a super verbal baby/child (and still is...scored 99.9+ centile on IQ) but no one outside of the three of us ever heard her speak.
Now, at the age of 12, she will speak in small, short sentences to a few friends. Her track coach of two years has never heard her talk. But, she has one friend who hears her as we do...non stop, constant, funny, articulate, sarcastic, enthusiastic etc.
She also has a DX of Aspergers, Social Anxiety and an IQ of 170 (extended norms). We tend to work a lot more on the social anxiety and dealing with every day situations like trying to order her own food in restaurants, paying at shop counters etc. We do not focus on the mUtism as it draws attention to herself and she feels she is letting everyone down.
Remove all the pressure to speak, let her speak through you and you may see some improvements.
However, we are now at the stage where at 12, this is affecting her whole life and she is missing opportunities due to not talking or being able to participate in activities. She refuses to go to therapy as 'why would you send me to therapy to do something I don't want to do...I don't want to talk' but in your case
I would be seeking therapy rather speech therapy, before she decides to refuse to speak to anyone about it.
