A writer's voice bubbles over with their feelings about their topic. Change those feelings and the voice used changes with it. Sometimes its as easy as making a decision. I recently heard one author say he had found himself using the word “seedy” to describe inner cities too often. He stopped and took the time to imagine how that would affect a reader living in an inner city. Now he makes certain to never use that or related words, and notes that consciously seeking better ways to describe city life has changed the way he writes.
Voice changes naturally when writers have a specific audience in mind. Think of your speaking voice. I bet it changes naturally depending on your audience, just like mine does. I can baby talk with the best of them when facing an infant or toddler. A teen or tween, an acquaintance or friend, coworker or boss, to a relative about the boss all require different voices. When I concentrate on who my listener is, I code switch without needing to consciously think about the change.
A writer's voice is the sum of the choices he or she makes while communicating information about a subject, and information about themselves, to a particular audience. Personality, attitudes, beliefs, and feelings about things, all being conveyed to a certain audience. Mainstream authors who want to write inclusively should first work to develop an inclusive Voice. It’s not enough to just want to write about diverse characters. An author must accurately see, hear and feel the world their diverse characters inhabit. If they can’t do that, their reader won’t be able to either.