I've been told that it's likely that most people who could read still read out loud in the not-so-distant past. For instance, I've heard it said that the Apostle Paul knew what the Ethopian eunuch was reading in his chariot as he went along the road because the eunuch was almost certainly reading aloud to himself.
We still teach children to read aloud, and it takes a bit of effort for them to switch over to reading silently, often with an intermediate phase of silently moving the lips.
For some of us it's quite literal, and goes by the term subvocalization.
I know that I, for one, when speaking to myself, sometimes am doing it through little breathy throat quasi-vocalizations, like I'm beginning to say a word that I'm mentally saying.
I wonder at my ability to carry on an internal mental monolog without it.