I definitely have an inner monologue! I have always talked to myself (aloud/muttered under my breath). As for just the amorphous voice in my head, I don't know that it was always as active, but I think as I started to text more and converse with people online as a school aged person, it probably became more well formed. And also after lots of therapy, I definitely talk to myself even more. The tricky part is when I have a full on conversation with myself and then I can't remember if that was only with myself or if I had told those things to the person I meant to have the conversation with in the end!
These are the two podcasts I always send when I'm thinking about this:
This is from This American Life, and one of the producers? Or staff doesn't have that inner monologue
https://www.thisamericanlife.org/731/what-lies-beneath-2021/act-two
Then this Hidden Brain podcast looks at how to harness? (Or maybe just live peacefully with) your inner voice
https://hiddenbrain.org/podcast/you-2-0-befriending-your-inner-voice/
For me, it's like sometimes I'm just hearing myself talk to myself but in my head instead of being muttered aloud, but sometimes it's just this sensation of hearing myself, but I'm just thinking thoughts. I don't hear anything exactly, but it definitely has the cadence of my voice for the most part. I wonder if it would be better or worse if my inner voice could talk faster. I already have some anxiety, so that would be a whole new dimension when it comes to racing thoughts!