I get this too. While awake, the sounds I hear on my own mind are just basically the signs I could make with my own voice. I can imagine a song by imagining humming or singing it, but I cannot recall what the actual recording sounds like. Yet there's that switch that flips as I go to sleep that I notice occasionally where I can hear crystal clear music. It's beautiful and I can't recognize the music.

I also get this, much more rarely, with images. While awake, I can't truly picture what I imagine, it's like I just have a notion of what is there, like motion and shape, without really seeing it, or at best a momentary flash of a picture. But when drifting to sleep there is a moment where the images become real, with color and texture and detail far beyond what I can imagine while awake. It's shocking and wakes we up when I notice it happening.

What is this switch? How can my brain have such discretely distinct modes of operation? Falling asleep itself seems so much more of a continuum.