so about half the time my "internal monologue" is my own voice giving myself instructions (ex. "ok, i have ten minutes to spare. what should i do? i think i'm gonna pull up pi.fyi and look at posts until i have to get back to work."). it's not literally me listening to my own voice because i am conceptualizing the voice much faster than i (or any human) is capable of speaking, but it definitely coherently exists in my head as a fleshy .mp3 file, and it is definitely my voice.
the other half of the time i am simulating social scenarios in my head. there are a variety of scenarios where i do this: sometimes i am actually rehearsing for a real social situation that is actually going to happen, sometimes i'm merely preparing for the possibility of a social situation, sometimes i'm trying to do a post-mortem on a situation that has already occurred so i can think about how to improve, and sometimes i'm just talking to an imaginary adversary in my head in order to find and plug holes in my worldview / philosophy etc.
adding the caveat that if i am alone (or merely think i am alone) the "internal" monologue / conversation frequently becomes an external one, and i am sometimes ambushed by my partner and / or roommates who occasionally hear me passionately arguing with an imaginary person and i've just kind of made peace with the fact that i am going to appear casually schizophrenic to anyone who catches me doing this because i genuinely believe that actively trying to rehearse conversations and arguments in advance is what helped me get over my debilitating introversion to the extent that i have