The psychological phenomenon of your mind turning a pretty mediocre song into a much better song when you recall the song from memory. Disappointment is guaranteed when you actually hear a song again.
Guy: This song sucks...I'm suffering from a case of aural transmutation!
Girl: What's that?
Guy: I remember this song being a lot better in my head. My mind must have aurally transmuted it into a better song.
Girl: Don't feel bad, everybody has to do that with Justin Beiber's "My World"