Language of thought hypothesis
[en.m.wikipedia.org]

MENTALESE: THE CONFLICT BETWEEN LANGUAGE AND THOUGHT
[researchgate.net]

Yes and it doesn't shut off except when meditating,drifting into or out of sleep, and focus demanding motor tasks. (Ironically, fast reading is one)

I hear myself frequently

I definitely have an inner voice, but because I have that voice I might not fully acknowledge how often I have thoughts without putting language to it. Especially complex thoughts seem to always have my voice accompanying them, but when I need less structure in my thinking I feel like I'm using little or no inner voice. If I'm crafting a sentence and editing what I want to convey, I have a strong inner voice. If I'm weighing options at the grocery store, though, comparing price and ripeness and whether the packaging is reusable/resealable, etc., ultimately coming to a decision, I don't think I really rely on that inner voice. I'll have to pay more attention to get a good handle on it, I guess, but I've always known that there's more or less of that inner voice depending on what I'm doing or the type of thoughts I'm having. In general, though, I use that voice a lot.

One of the comments rang true for me. Some of my friends cannot be silent with thoughts for long -- have to talk every moment. I have never been inclined to talk every moment. I filter my thoughts before I vocalize them to others. I wonder how constantly-talking people can filter their thoughts. My inner dialogue echoes my emotion, conscience, fear, love, desire, etc., and the rational part of me brings sense and order to all those elements in chaos. If I were to speak without contemplating first, I would express chaos probably.

You can gag your mouth but how do you gag your brain? Posting screenshots; hell my house is littered with so many notes I have had to organize and categorize them. One of the worst time is very early in the AM. If I have to get up to pee then it starts. Have to make a strong conscious effort to be quiet otherwise I will be awake the rest of the morning.

I think one problem is having too much time on one's hands. In this time of year there is not so much to occupy our bodies never-mind our brains so we wander and daydream.

When my late partner was dying she had a problem with reading. She was an elementary school teacher and when kids first start to read they verbalize the words. She had to train them to read silently in their minds. When she developed a brain tumor she lost that ability. She said she could read but would have to do it verbally.

Absolutely. And it won't shut up.

Yes. Sometimes more than one. Sound effects. Visual effects. Tactile effects. And sometimes they leak outside my head and I can see/hear them in front of me as an overlay on top of the material world. Doesn’t everyone?

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Maybe related; maybe not:

Formal Operational Stage

"The research demonstrates to us that only a few (about 10%) of the adult population are ever fully equipped with formal operational thinking skills – whether or not they have received any higher education. Between 30% and 40% of the population lack the ability to engage in this type of thought completely. The remaining 50 to 60 percent have some formal operational thinking skills ranging from barely demonstrating that they have any to usually, but not always using them."
[hethoughts.wordpress.com]

Wow! I thought everyone heard the voice! I wish I could shut it down sometimes, but I can't.