Yeah, this is crazy to me. (I'm hearing these words in my head as I'm typing them.) Like, what form do your shower thoughts take if you don't have an internal monologue?
To use a software analogy, thoughts are the model. English words are one possible serialization.
(Other possible serializations include other phrasings of English words, Japanese words, mathematical formulas, drawings, musical notation, and so on.)
It's nonsensical to ask me for a more fundamental form of a thought in my brain. It'd be like having a program where you have a reference to a directed graph in memory, and asking "So what form does this graph take? Like XML, or JSON, or what?" It doesn't have any other form. I could serialize it into one of those forms, if need be, but right now it's just a bunch of connections in memory.
I think I can produce the same voiceless thoughts. But my voiced thoughts are crisp, clear and easily hold in memory compared to other modes of thinking.
I wonder, maybe that's why I'm excellent at talking about problems, understanding them at high level, but have so much trouble with acutal engineering.
When I have verbal shower thoughts, they come out verbally and I sing or monologue.