I am having this reaction also.

I am literally speechless, thinking that it wasn't an idiom; and that the monologue as recorded in e.g. modern literature, was likely meant to be read and as is read as a transcription...

I have always at a deeply unexamined level assumed that that was just a convention of how literature functions through its medium, language; that the recording of thought through its semantic content was a (to me "obvious") necessary translation-layer thing.

I don't have a headache, but I might have to go sit somewhere and stare with my eyes unfocused for a while, to come to terms with this. I've been cross examining my coworker (who does "hear the voice") about what that even means.

The aphantasia thing was curious. This is somehow much more of a shock.

Yes, for example there's the movie trope of becoming able to hear "the thoughts of others", and then the person goes in a bar and hears these sentences, like "hm I wonder if she ..." and so on.

I pointed out how unrealistic this is and you obviously couldn't hear such things, as thoughts aren't literally sentences like this. And my friend was just brushing my comment off as if I was not making any sense, since thoughts are obviously sentences like this (for him)...

Yes, I always thought that when a novelist describes the “voice in the head” of a character, or when a coworker does the same, they are trying to convey thought over a medium incapable of conveying true thought.

I never imagined that that was the actual thought!

Do you not hear music in your head? Like I have a song running through my mind right now, lyrics and all. No visuals just the music. If you don’t hear a voice in your head wouldn’t that mean you can’t ever hear music in your head either?

Other person here: I can only hear realistic sounds and visualize realistic images when I'm in a half-asleep state or in dreams. Otherwise it's very blunt. For example I cannot realistically hear music in my mind, I can imagine myself humming it though (and my vocal muscles ever so slightly tense up if I do), and I can imagine myself speaking in the same way, but it's mostly a very faint thing, and is mostly about imagined movement of my mouth etc., rather than sound. E.g. imagine clapping your hands as a motoric action, but imagine them in your hand, not the visual of it, but imagine what it would feel like in your arms and hands. It's a bit going in that direction with my imagined speech.

Similarly until last year I never knew that people actually could visualize things when they closed their eyes. I thought it was more of a metaphor. But alas when I close my eyes it's darkness. Thankfully I have my internal monologue to describe things to me in that darkness.

When I close my eyes the images I can produce are no more vivid than the images I can produce with my eyes open. E.g., I can picture the car driving down the street behind me, or I can imagine a sphere in front of me. But in either can it's semi-transparent. It's like a hologram places in 3d-space relative to my-self, and it doesn't matter if my eyes are open or not. This is what I consider imagination. I thought this is how it worked for everyone.

How does this relate to what you're saying? Are you considering this visualizing things or not?

I can't visualize every kind of object. The easiest things to visualize are simple geometric shapes with a strong color. By default, if I try to "visualize something" it will be a red triangle.

I cannot visualize faces whatsoever, even if the person is right in front of me and I close my eyes suddenly.

Glad it's not just me that cannot visualize faces whatsoever! But with the exception of faces, I can visualize complex 3d shapes if I'm lucid dreaming. When I'm fully awake, my visualization abilities decrease quite a bit, so when I get a chance (which isn't so often with kids) I'll often lie in bed for an hour or so just thinking visually. My wife thinks I'm being lazy, but it's often the most productive part of my day.

Have you tested yourself for face-blindness or prosopagnosia?

For me, thinking verbally means either monologue by someone else than me, or dialogue with not-me. Rarely dialogue where neither party is me. You've never argued with ancient Greeks? I find the act of dialogue with my idea-form of Plato hilarious.