Nanoro, if I have read what you are asking correctly (to change your voice pitch permanently), Magick won't help with what you are looking to do. It can't change any of your physical attributes as it works within the boundaries of this physical plane.
Training your voice is the only thing that will help. Unless you go for a more medical approach. Help yourself till there is no other way, till you are lost than ask for help. Magick can't be the first thing you resort to.