I'll take your questions in reverse order. (Note: Wikipedia gives audiation a purely musical meaning, but I'm using it here to include hearing speaking voices and other nonmusical things in one's head.)

I have what I think is above-average skill at visualization and audiation. (For musical things, my skill is definitely above average even among the musicians I know.) During the day, these are both under my conscious control -- I decide to see or hear something in my head, and I do.

When I'm falling asleep, though, my visualizations and audiations become

• much more vivid, and
• spontaneous, in the sense that even though I still have some control over them, if I'm not exerting control at the moment, they keep going (like I have a TV station in my head that broadcasts nonsense).

Despite the vividness and spontaneity, though, I never think any of this is real. It has a clear quality of coming from inside my head rather than the external world.

In the very rare occurrences of actual hallucinations, which are always voices and usually saying my name, they seem real. The only way I know they're not real is context -- I live alone so I figure it's more likely that I'm hallucinating than that someone has entered my house late at night and is talking to me. (Back when I lived with someone I would holler back if it sounded like someone was calling me from another room.)