Voice, to the extent that it is a thing in itself, is not a static or stable thing. A writer’s voice changes with time, and all the time. With every piece, every character, every subject, perhaps with every line, a writer’s voice has to change. I would worry that writers whose voices do not change over time would get tired of hearing themselves talk. This is not to say that that voice is fully and wholly new each time a writer writes; but each time a writer changes, so too does her voice.