For me it was from introversion (which I understand may go along with perfectionism - at some point the line starts to blur).
I've probably shared this story before, but when I was in fifth grade, over a period of time, I was not responding loudly enough to answer questions. It was a combined classroom of two teachers, 60 kids. They made me stand on my chair in an effort to make me respond louder, to somehow rid me of the shyness. Well, it sure backfired - it left quite a scar. I'm still mad about it all these years later I guess! I even remember what I was wearing - it was a jumper in a stiff tan-colored denim-like fabric, with boots LOL (circa 1978). A skirt, standing on a chair, what in the world were they thinking.
So, if your dd is experiencing anything like I was, the last thing I'd suggest would be to force it. As a practical matter, a little "I'm sorry, I didn't hear you dear, can you speak up?" is about as far as I would go in a group scenario. (Perhaps the embarrassment of being called out on the quiet speaking is enough to encourage her to get it loud enough on the first try next time.)
Is it normal? I don't know (quite possibly I was abnormal LOL) but it didn't stop me from growing up to argue in court, etc. Well ok it almost did but in that case, I was able to overcome the fears (a) due to lots of substantive preparation so that what I was saying was correct and I could back it up, and (b) because I was the one forcing myself to overcome it as a means to an end, and no one else, especially no teacher or parent, was doing the forcing. (I'm still angry with my mother over related things but we won't go there right now.)
that's my two cents on low speaking in school
