for me it's literally just my own voice (and i can mimic other voices) and i sort of hear it overlaid on everything else, sorta like what happens when you hold a hand in front of only one eye, you see both things at once.
The only real difference from my physical voice is that there's none of that delay and friction from having to physically speak, so it's harder to prevent my inner monologue from saying stuff which is extremely annoying since i'm autistic and thus have echolalia, meaning i just reflexively repeat things i hear..
The idea that other people have some sort of independent inner voice, or even several ones, is rather terrifying to me.