Depends on the context. I've stopped listening to music on my own accord years ago, so that is largely out. Although I do hum tunes, or more particularly rhythms, to myself. Most of my actual thoughts however are a dialogue, as if I'm talking to someone. This happens when I have time to be lost in thought, and it's generally me explaining something. If I'm trying to understand a physical occurrence, like electricity or hydrodynamics, I'll generally explain it to myself while visualizing it. Then there's reactionary thoughts, like "Fucking Hell" and "God Damned Mother Fucker." This usually happens during particularly harsh activities which utilize your hands. Then there's silent observation. I'm not saying anything in my head, nor am I visualizing anything. I'm just watching the events play out before me with curiosity.