FYI that process is called "subvocalization" - I'm only passingly familiar, but if you search that term there's lots of literature about it.
"This inner speech is characterized by minuscule movements in the larynx and other muscles involved in the articulation of speech. Most of these movements are undetectable (without the aid of machines) by the person who is reading."
The worst thing about the inner voice is it's not just when reading, it won't be quiet at bed time. And I have a tendency to complete thoughts as words. Even if I know where it is leading, and what the conclusion is, it feels like I have to see it through and phrase it correctly in my head. I feel very uncomfortable leaving it hanging.