She is beautiful! I hope your new vet can help her. She could be sleeping so much if she's not getting enough oxygen. I hope it's something easy to fix for your baby.
As for silent meows, I have one that is silent inside and one outside. I also have one inside that you can hear all over the house!
They are all ferals. The silent one inside has been vocal a couple times but his loudest was when I came home from work to a small fire in the house. He was meowing for some help then!
Good luck to your precious baby! Prayers for her.