I have these too but they are auditory, and they happen when I'm falling asleep or waking up.
When falling asleep I often imagine the sound of random conversations or bits of dialogue. The sounds are clearly in my mind, not my "ears" and it is often the voices of characters I've heard on TV (e.g., once I "heard" Monica and Chandler from "Friends" arguing and joking as if the TV was on in my head). It's like I'm hearing the dialogue of a dream without any pictures starting yet.
When fully asleep I often have dreams that are entirely auditory with no pictures. Sometimes I write essays on a typewriter in my dreams, hearing myself proofread the paragraphs in my mind. I've written songs as well but they normally disintegrate within seconds of waking up. The other day I wrote one down. It was a bit of a song that my mind wrote. It was sung in tune as: "I have a picture of you, to soften the process of what I went through". LOL. Lame but curious. Again, no pictures.
Waking up I often hear telephones or doorbells or someone calling my name. I've grown so used to it that I can dismiss it instantly.
I had a sleep study and was told this is hypnogogia and hypnopompia like you said. It is not considered schizophrenia despite hearing voices, because it's only associated with sleep. The sleep study also said I don't get to the deepest stage of sleep. I forget what it's called but it's one stage deeper than REM. I also have apnea. Not sure if it's all connected.
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