As some of you may know, I do baby music classes. I am doing some postgrad work on older 2-8 children and did my first research class this morning.
The children the staff had identified as shy and not speaking, all sang and joined in. Cue amazed staff.
obviously early days but initial theories are:
I wasn't self conscious about using music so they mirrored me.
Singing is less threatening than speaking.
I kept it simple and didn't frighten them with music that was too complex, too fast, too loud. I mostly used my voice not a CD.
The loud, confident children sat still, meaning the shy ones could have a voice.
Anyway, I hope this helps.