In a quiet, empty room with no external sound source present, a patient clearly and repeatedly hears a voice commenting on his actions, experienced with the same sense of reality as an ordinary sound, while he otherwise perceives the room around him accurately. What is this?

This is a hallucination: a perception that arises with no external stimulus at all, here in a genuinely quiet, empty room, yet experienced with the same sense of reality as an ordinary sensation, while his perception of the actual environment otherwise remains intact. It is not an illusion, in which a real stimulus is present but misread; here there is no real auditory stimulus being misinterpreted, since the room is genuinely quiet. It is not a vivid memory, which the person experiences as self-generated and past, not as a real-time external perception. And it is not an act of deliberate imagination, which the person recognises as self-generated and under their own control, unlike the involuntary, externally-perceived voice described here.