Part of the show Neuroimaging
Why do I talk out loud when I dream? My wife hears only half the conversation, and I don't even remember it.
My own view would be that the nervous system suppresses the motor system when we actually go to sleep. This is why we don't end up acting out more of our dreams, because of this region in the brain stem that stops the motor pathways going through. That will include some of the more complicated movements that make vocalisations happen, but you’re still, in your dream, going to be talking and things, aren’t you? So, that’s why you would talk out loud sometimes - I guess there’d be a breakthrough of that suppression activity.