I can apply any character voice I can imagine to the inner voice. The default that I "hear" most of the time isn't even my voice - the timbre is a bit lower and more neutral, and it lacks my distinctive vocal affectations. But if I want to hear it as an Irish woman, or whatever else, I just do. Perhaps it's relevant that I always did character voices and accents out loud as well, since I was a kid wanting to be an actor.