OK, I have one more question for you guys,.

It seems that DD is speaking often at a very low volume at the school. So low that it is almost impossible to understand her. She is not doing all the time, but apparently quite often. I do not know if this has been always like that or it has become worse at the time. She does not look at all like shy and she participates in group activities according to the teacher.
At home she has a perfectly normal volume, and an excellent diction. In fact she always was like that. When she was small (3yr) people would comment on how good she did articulate words even though they could not understand Spanish.
With other friends as well there is no problem.

Now that I think of it, she tends to speak lower when she meets someone for the first time.

Anyway, as soon as she sets foot on the classroom, one can barely hear her and when she addresses the teacher is even worse. I have seen as well other class mates having difficulties hearing her. Not all the time though.
When she brings friends at home there is not problem. And I think it is the same at other's houses.

When I tell her that she has to speak louder she becomes very distressed and insists that her volume is adequate

Is that 'normal' or should I become worried?