7 year old whispers

My daughter is 7 and can speak in a normal voice when it is only me or one of her sisters in the room. Whenever others are in the room, she will only speak in a whisper.

She has done this since she started talking. There was no speech delay. When I ask her why she won't speak louder, she just cries and says she doesn't know why.

When she started school, she refused to speak with any teachers or adults in the school. After almost 2 years she started speaking, but again will only whisper. She speaks clearly when she is playing with other kids. When her Dad or any other family members are around, again only whispering.

I tried to speak to a doctor about this with the support of her school, but when I took her in to see the doctor, she spoke to him and answered all of his questions in a normal talking voice and made me look like an idiot.

Her learning is being affected as the school is unable to assess her as she mostly refuses to speak. She is in a class of 28 and I feel the teacher doesn't have the time to spend on her to build her confidence.

I am unsure if this is something that will remedy over time or if it is something she needs help with. I do feel this is more psychological than behavioral, but I'm not a professional.

What do you think I should do? How do I get professionals to really listen to what I am explaining to them?
Regards
Laura