I have a student who has this issue as well, and this is my third year having her. She started out very quiet and would only talk when she wanted something. She loves birds though and considers her toy birds to be real. I would talk to her birds, ask about them, and as I invested more of my interest in her interests the more she came out of her shell. My suggestion is find something that the child is interested in and dive into it, whether it's trains or animals or Super Mario Brothers, it shows you're interested in them and I think for my girl she wasn't used to teachers or people actually caring about what she wanted or liked. She was used to them telling her what to do and the mutism was a way of putting a wall up and controlling her environment a little bit.