A short time ago, I heard a critique about Max Brooks' fictional oral history, World War Z, that I have not forgotten: every quote sounded like it was coming from the same person. Even though dozens of people from across the globe are interviewed after this zombie apocalypse, all the quotes sound like they're from one person. I've kept that in mind as I've worked on the second book, but sometimes, oral histories can't help but sound like they're coming from one person, fictional or nonfictional.
Two books that I still cite as big stylistic influences for my book are Fool the World and The Other Hollywood. In particular, the flow of the quotes from person to person in Fool the World has been a big influence. With The Other Hollywood, the massive amounts of different people quoted yields to a handful of very fleshed-out characters.
In those books, the use of language might sound similar, but the different personalities come out the deeper the quotes go. I definitely got a strong sense of who Charles Thompson was, as well as Linda Lovelace. But as far as word choice from them, that seemed very secondary to me. I just wanted to read something that was compelling. And I must admit that I sped through Fool the World much faster than I did with The Other Hollywood.
Trying to make a twisty storyline with quotes from at least twenty different people sounds tough -- and it is -- but that's part of the fun challenge of writing the book. The lack of a "proper" narrative might sound like you're trying to walk straight without a spine, but if the quotes are good enough, the narrative is not necessary.
I think of this like a great documentary without any voiceover narration. If the quotes can speak well enough, then why use redundant commentary from somebody else? Sure you run the risk of sounding like the same person based on word choice, but that can be an inevitable drawback in doing an oral history.