pfffffffff. This is a great question but pretty hard to establish.
The meaning of words processed subliminally are valid. If you subliminally process DOG CAT MOUSE you are left with more than just the sounds, your brain processes
the meaning of the words also.
http://serendip.brynmawr.edu/bb/neuro/neuro99/ web2/Hellew.html
They are probably recorded even though you are asleep, but it may be difficult to record them since your mind needs to find some kind of route back when you are awake, and it did not record them when awake, so it has few cues to take you back to them. Tests can still be done to establish if the memories are there though (priming tests, forced-choice tests).
That's as far as I go I'm afraid, I worked in processing of subconscious memory for a while but never in sleep. Worth checking out though!