I have experienced both, the difference is how I hear it.

Hearing legitimate voices sound like they are coming from the outside, while intrusive thoughts have always sounded like thoughts to me on the inside of my head. Regardless of how "loud" my intrusive thoughts got, they never sounded like they were coming from the outside of my head.

My apologies if this is not what you were asking, I had a bit of a hard time understanding your question.