I have noticed also, that when going sleep, all types of sounds can appear to sound as music although a second ago it might have sounded like someone talking when you were awake, that same sound can turn into patterns of music while at sleep. It might have something to do also with electrical impulses in the brain that are changing when you go into the sleeping state. I have heard music a lot in my sleep of all kinds that I remember when I wake up, but am a musician so perhaps it's to help me write music. I have also heard music that sounds like it comes from above, it's so incredibly beautiful, beyond words! especially many times when I fainted often years ago, but also sometimes when I sleep.