My internal voice is pretty much the same all the time when reading, and isn't very voice-like. Words, yes, but without a great deal of vocal character.
Occasionally, if it's a book I associate with a particular performance, I might hear that actor's voice when reading, but not always even then.
One thing I think is fascinating is that not reading aloud was once quite unusual. The classic text that shows that silent reading used to be unusual is in
Book 6, Chapter 3 of the Confessions of St Augustine, where he talks about St Ambrose:
"Now, as he read, his eyes glanced over the pages and his heart searched out the sense, but his voice and tongue were silent."
There can be no reason for St Augustine to include this sentence unless silent reading was unusual.