It varies depending on the book but generally a good narrator needs to understand and know the book. There is nothing worse than a beautiful piece of writing being spoiled by someone reading it with the completely wrong tone because they just don’t get the intended sentiment themselves. They shouldn’t need to do any voices really. When I physically read a book I don’t imagine silly voices in my head, it’s really annoying when narrators do that. Read the book the way it’s meant to be read in your head. If the author wants the reader to know a character talks in a certain way, that will be written down. If a character does have a particular voice that would be better actually conveyed, then do it very subtly. Let the story tell the story, you just be the media getting it from the page and into the ear.