Occasionally, when I'm like definitely alone I'll talk out loud to practice explaining something or just go over something, which for some reason I can't do clearly in my head. Especially math. XD
Mumbling? Eh, I guess I do sometimes, but I don't even realize it. If someone is in close vicinity I'll just assume they can hear me, which apparently isn't the case in places like a sports game or cafeteria, haha.