It's like a full on multi-sensory jungle inside my head 
I find it amazing that I can drive along, paying attention to the road, sing to the radio on autopilot and have another stream of conciousness rambling on through my head. That's at least 3 different simultaneous mental processes.
My internal monologue is pretty incessant. I can also visualise things clearly. I used to get into trouble at school for giggling a lot for no apparent reason, usually for some thought that flitted through my head, often kind of animated on top of actual vision like Mary Poppins.
Sometimes I'll lose a word and get stuck on the first letter. I was trying to remember a name the other day. I knew it was a C. Callum kept coming into my head but I knew that was wrong... it was Caleb so there was some logic going on there.
I'm scatty and find organisation a battle, not because I forget things, more that I don't remember at the right moment for action. I need tight external deadlines to focus my brain. If my mind is particularly busy, it's a bit like that scene in Harry Potter where he has to find and catch the flying key amongst all the other keys.
It is quite fast paced in my head, and I find slow, predictable speakers very frustrating because I have to wait an extra 3 seconds for them to finish saying the obvious before I can respond. I would get the flow of dialogue through the supermarket, but I am fairly descive, and want to march through the place and get the job done.
I hated silence as a child, it was incredibly distracting and I needed music or background noise to focus. I cope with it much better now, but do have the radio on for much of the day.
DS has autism and dyslexia. He has a strong internal monologue. I asked him after a similar MN thread and he seemed relieved to acknowledge the voice. He had worried that his teacher would tell him off for not being quiet when his internal voice was talking away. He says he can visualise things like a film too.
I can believe there are people without an internal monologue, and have taught pupils (teenagers) that didn't seem to be able to think without verbalising every little detail that passed through their head. I have also encountered pupils that just didn't seem to think much, and that is quite different to the much more common day dreamers who are clearly focused on something else processing through their heads. (I was a doodler, or off on my own tangent but clearly simultaneously engaged with what was going on as I'd have my hand up Hermione Granger style and rarely missed important information)
I think it's clear why I rarely post a brief response on MN 