There's actually quite a variety of what goes on in people's minds. Some people's internal worlds are similar to each others', and some are drastically different. I think of it as one of the cool things about being a social species: people think differently, so we can solve different problems to benefit the group. I think that as a species, some of the worst things happen when we all think alike because people who all think exactly alike are easy to control.

How do they read?

The words I read make sounds in my head but when I'm just thinking I don't have any inner monologue, I just kind of think the thing directly instead of vocalizing it in my head.

And they can't even talk to themselves in a vampire voice.

I can kind of shut off the voice when I'm reading, so I just look at the word and know what it is. It's harder with unfamiliar words, those have to be sounded out.

I always thought "inner voice" was a figure of speech

I have an inner monologue, but it isn't active when reading (besides, perhaps, to critique). I just see symbols and know what they mean, I don't have to say the word in my head.