I am communicating with myself; when I read I basically verbalize in-my-head what I am reading (and what I am typing). It mostly sounds like my physical voice but sometimes it could be in another's voice depending on the context and situation.

As far as to "whom" it's like talking into a room with an empty audience, sometimes I can visualize an audience to talk too and make-up things / situations for them to say but most often it's just me.

I am genuinely curious how you actually plan-ahead without having an inner voice, do you just "talk" to people without verbalizing it internally?

As to "why" I can't explain, it's been there since as long as I can remember... perhaps the voice has gotten louder over the years as I have learned to do "more" with it; I work as a Software Engineer day-to-day so most of my day is spent building mental structures and models of applications in my head and walking through where I'll do certain things next or even talking with my inner voice about said things in a form of rubber duck debugging.

Even this post, and your post are basically read back aloud and if I knew what you sounded like I would likely read the post back in your actual voice.

Without my inner voice... I don't think I would feel like I exist as a person, the bones / muscle / flesh surrounding my body are just what give me mobility but that "voice" is "me".

Which is why perhaps when I say you could clone the brain, since you can't clone my inner voice "I" will cease to exist. To my friends and family I might still exist but it'll be a different "me".