There are many theories as to the cause of hypnagogics, whether auditory or visual. The preferred one is that our brains and/or minds go through what they've experienced (heard/seen) and process them. I can testify to the veracity of a good portion of this. I have heard 'replays' of commercials, songs, and things people have said to me. Then there is the Akashic Record/Collective Unconscious theory, which I also subscribe to, in which our mind/brain 'tunes in' to frequencies in the ether (excitable space, astral, etc.) which are experiences that others have had and are imprinted. I can say that I've received info that I couldn't know otherwise that way, but frankly, not that often. Then there is the alternative universe/higher self theory, which I also subscribe to, which points us to 'tuning into' other lives we're having, either a higher self or a portion (individual incarnation), possibly one that is close enough to this one to be easily connected to.
I personally think that we do all of the above, in total.