It wasn't a conscious decision, I remember noticing that I had no inner monologue continually, it'd almost never stop. Years later, I realized I just didn't speak out loud in my head except for a few occasions.

I think it may be related to the fact that I don't subvocalize when reading, maybe. Even right now, when thinking about this reply, I can feel what I want to say forming in my mind, but it's not words, it's just a concept and then I put it in words while typing it.

I'm sorry I couldn't be more useful, alas.