Or the italics make it clear one voice is out loud and the other consists of voices in his head. I do agree there would need to be more context to make it the most clear.
The hyphen and extra indent do that already. Adding italics to it is just a bit much. Readers only need one good cue to understand that it's his subconscious; using three can get frustrating, especially if this sort of "dialogue" happens a lot.
To you that might convey the second voice was in his head, I don't doubt that. To others, italics is the convention for internal monologue, as you can see here in this 41 page thread.
I do agree that italics is the best way to convey direct internal monologue or thoughts (assuming it doesn't last paragraph after paragraph!). What I'm saying is if a person uses so many cues, it can get frustrating to the reader. I've read books where an author really slaps the readers with information to make sure we got it, and it came out as irritating. The reason I didn't suggest using only italics is because from my research, most people don't think italics should be used as often as this story likely requires. A little every now and then is acceptable, but using italics constantly seems to frustrate people (myself included).
