I don't know about hearing voices in your head, but I know dementia can cause delusions. My grandma has them occasionally. I have no idea what the heck "mood swings" is supposed to mean? He thinks that your dad is bipolar? Sounds like the doctor doesn't know what he's talking about. If you can find another doctor, I would.

I really hope that he can be diagnosed soon. Still sending prayers and love to your dad, you & your mom.