Believe it or not, I am no expert in vocal cords and vocal pitch. I am mathematically trained, and I work in numbers. These are questions you might want to pose to an expert.
As for my story, I do prefer to keep science out of it. At its core, Brothers Apart and all subsequent AUs are based in the supernatural, with witches and demons and curses that shrink people down. I personally despise the ‘squeaky voice’ trope, so if you read a story by me you can assume all characters sound ‘normal.’ Whether this is me taking liberties is up to you, but until we actually have a person cursed to under four inches tall, none of us are the experts.