I know everyone wants to create their own acronymn. But to my knowledge there is nothing to address hypnagogic SOUNDS.
You would think that it does not matter because (HI) includes sounds. Well that may be so.
I usually have the standard (if there is such a thing) hynagogic imagery. Images randomely appear along with sounds too.

Hynagogic, meaning relating to the images or hallucinations sometimes perceived during the state of drowsiness actually mentions nothing of sound.

I am not sure if other people have these experiances or not.
I have stents where I have no visual input. It is all auditory. It is as fabulous as images. Once you let them flow and you begin to drift the sounds become increasingly more clear. If you can maintain you awareness you can manipulate whatever sounds you are hearing and be you own conductor.
I can at times lead this into a lucid dream. As of late I have had little success. But clear hynagogic..... sounds.