Not so much based on, but rather completely plagerising Kate Nash's 'Mariella'. Which is a fantastic song, you should listen to it.
When Sherlock was 5 years old, he glued his lips together.
'If you have nothing nice to say, don't say anything at all.' His mother had told him, repeatedly. And his Nanny. And his teacher. Then even Mycroft had said it. But he never did have anything nice to say, he had the truth and what he could see about people. But nothing nice. So he decided to stop speaking.
Then he threw away all his clothes, all his costumes and hats, until he was left with a wardrobe of black. Lovely mysterious black, maybe when he grew up he would have cheekbones like Daddy and look really cool.
Of course the children at school thought he was strange, but they already thought that. In fact, they preferred this new silent Sherlock, at least he wasn't calling them idiots anymore. His teacher rang home, assured Mummy that it was just a phase and his work showed he was still far above average, and if he just showed some willingness to understand rules and interact properly with others maybe he could move to a higher class.
'Sherlock, darling, stop this silliness.' Mummy had come to him after a couple of months of silence. 'Unglue your lips, wear the lovely purple shirt I bought you. We can invite some of your school friends round for tea. Please Sherlock.'
He had scowled, crossed his arms, and walked upstairs to his room. He had sat at his desk, patted his skull and thought I don't have friends, I don't need friends to do my experiments and I don't need to talk to think.
Sherlock didn't mind being alone, he liked not talking. Sometimes people thought because he couldn't talk he couldn't hear. People were stupid, but it let him hear things he shouldn't, find out secrets he wasn't supposed to know.
And he still did the crosswords every day, the easy one and the cryptic one. The cryptic one wasn't really that much harder, but other people seemed to think it was and they are always impressed when he does it.
So he is happy, in his world full of secrets and puzzles and generally being better than everyone else. He doesn't skip or dance, because he isn't a girl, but he does sometimes smirk to himself and have a little happy jump (but only if he is absolutely sure no one is watching).
Then Mycroft comes home. He is sat on Sherlock's bed when he gets home from school, with that stupid purple shirt Mummy bought in his hands.
'Such a shame you're only wearing black these days.' He says, not even a hello to start. And people think Sherlock is rude. 'This really would look nice on you. And you've stopped speaking. Do you plan to start again?' Sherlock shakes his head hard enough that his hair slaps into his face. 'What, never ever ever ever ever ever ever? That is a problem. How on earth are you going to tell me all the secrets you've found out and the deductions you made?'
Sherlock looked at the floor briefly and bit his lip, before turning back to his brother, mouth opening to spill all that he had learned.
The next day, he went down to breakfast wearing the purple shirt.