I'm not so sure there's a big difference in voice between MG and YA. I mean, the voice of Harry Potter doesn't change that much from book 1 to book 7. What makes the later ones YA is the content--sexual attraction and murder. But even then, the distinction of calling it YA comes from other people, not the author. She's been quoted as saying that she wrote the whole thing for the same group of readers--ages 10-14. (No, I don't remember where I saw that quote.)
I also think the distinction between MG and YA is more important to us--writers, agents, editors, publishers--than it is to readers. I don't know that I've ever seen a Middle Grade shelf in a bookstore. They have all sorts of other labels--Classics, Fantasy, Independent Readers, Series, New Releases, Contemporary, etc. Or just Children and Teens.
I've also seen a lot of books that are shelved in Childrens at Borders put in Young Adult at my library. So... I think it's easy to get too stressed out about it, when you could just be focusing on the story.