Shadow Bonnie is a bit of an enigma, and he doesn't follow the same rules as the rest of the animatronics (mainly because he isn't one). He is a mysterious shadowy figure and should have an equally dark and ghostly voice to match. His voice is similar to Golden Freddy's and so is his way of speaking, so looking at that can help you get an idea for it. The difference is that Shadow Bonnie isn't a spooky cartoon character, he's just actually some kind of paranormal spirit.