Not all people do have an inner monolog when reading, called subvocalization. Subvocalization is when you are basically reading to yourself inside your head, sounding out each word. It is one of the most common reasons for slow reading speed. Most people do not “need” to subvocalize and can train themselves to process the visual text directly, instead of first converting it to “auditory” information.

I found this out a few years ago and I was shocked that the way I read wasn’t universal. I have since been practicing reducing/eliminating subvocalization and I am getting better, and it allows me to increase my reading speed significantly. It also serves as an excellent example of how different our internal mental processes can be, and how completely unaware we are that there could be any other way to think than our own.

Don't we all experience this from time to time? When I'm focused on solving some mathematical problems I'm not thinking in words, but in concepts. When you are thinking of words you also think of a concept, the only difference is that sometimes there are no words associated to it. Im my opinion, words, sentences are just a label to the thinking process, a translation of what is really going on inside, not the driver of it.

That's true - though I think of an inner monologue as being more "self driven". Perhaps it's just that their mental voices don't spontaneously say anything.