Micro-muttering

Speaking in a very inaudible voice to oneself within earshot of others. People around you ask, "Are you talking to me?" a couple of times since you are having your own back-and-forth conversation; one voice out loud, but not loud and the other, in one's head.

Billy, are talking or listening to me? I asked you twice to speak up. Oh sorry dear, I was simply micro-muttering.