I do have an inner voice, though not, I think, as insistent as some here describe. I (seem to) think in words and I do rehearse in my head what I’m going to say or write. I say “seem to think in words,” as I suspect that’s a high-level process organizing and self-justifying what lower-level parts of my brain have already produced and decided, bringing it to my “consciousness.”
Something I’ve started to notice more recently, possibly related to not getting enough sleep: If I’m relaxing and nodding off, I’ll start to get an internal monologue that I don’t feel in control over, it just comes. When I’m fully conscious I’m in complete control, but not on the edge of sleep.
When I read, I do sound out the words in my mind, but I also have, and feel I have to have, a strong visualization of what’s being described. When I’m doing math, I visualize what I’m doing at least as much as verbalize it; I have very strong internal visualization, which is possibly why I went into a field related to geometry. In fact, when I’m deep into working on math, I find I start to lose some of my verbal ability (start struggling for words, etc.), as if my mind is co-opting the verbal part of my brain to work on the math.
My wife, on the other hand, is much weaker at visualization than I am, which I’ve always found an interesting contrast. She’s a much faster reader than I am as well, now I’ll have to ask her if she sounds out words as she’s reading them or not.
Finally, I very often have musical earworms in my head. My mind is a very busy place sometimes.