Normally, any sound in my head is perceived as my own internal voice mimicking it. I can repeat tunes as well, but it is my own internal voice. I heard music one time while on mushrooms at the beach. It was in my head, and not created by my internal voice. It was as obvious of an "audio imagination" as the few time I have experienced "visual imagery." It feels like even with a lifetime of inability, when I experience the small glimpses mental imagery, it is OBVIOUS what is happening. To be clear, I am SEEING an image with my eyes closed that is as clear as open eyed daylight. Usually in the milliseconds that realization takes, the image disappears.
To link this back to imagined sounds. My consciousness of my personal internal voice is different from the audio hallucination that I described. I actually HEARD that music. I have a feeling that if I randomly experience imagined sound, it will be obviously not my internal voice making the sound.
Sometimes I think about people getting a song stuck in their head........are they HEARING that on repeat over and over???? damn. My experience is a compulsive feeling to keep "vocalizing" or "creating" in someway, they given stuck song.