play back vs replay

play back is not a noun while replay is a noun.

Word	Noun	Adjective	Verb	Adverb
play back	No	No	Yes	No
replay	Yes	No	Yes	No

As verbs, play back and replay are synonyms defined as:

play back (noun)	replay (noun)
	the immediate rebroadcast of some action (especially sports action) that has been recorded on videotape
	something (especially a game) that is played again

play back (verb)	replay (verb)
reproduce (a recording) on a recorder	play again
	repeat a game against the same opponent
	play (a melody) again
	reproduce (a recording) on a recorder