Well, anyone who can remember a vivid dream where multiple things were happening at once or where they were speaking or otherwise interacting with other dream figures whose theory of mind was inscrutable to them during the dream should clarify that the mind is quite capable of orchestrating far more "trains of thought" at once than whatever we directly experience as our own personal consciousness.

That would be my input for people to not have to experience schizophrenia directly in order to appreciate the concept of "multiple voices at once" within one's own mind.

Personally, my understanding is that our own experience of consciousness is that of a language-driven narrative (most frequently experienced as an internal monologue, though different people definitely experience this in different ways and at different times) only because that is how most of us have come to commit our personal experiences to long term memory, not because that was the sum total of all thoughts we were actually having.

So namely, any thoughts you had — including thoughts like how you chose to change your gait to avoid stepping on a rock long after it left the bottom of your visual field — that never make it to long term memory are by and large the ones which we wind up post facto calling "subconscious": that what is conscious is simply the thoughts we can recall having after the fact.