This is a very intellectual discussion between two people who have left the trivial worries of this world far away (or below? :))
Imagine yourself sitting in front of your comp, speakers blaring, your favourite pearl jam number flowing. You put a stop to whatever useless, productive activity you are doing, close your eyes, start singing along. You enjoy every note, every crooning in the song, and then you begin to hear nothing but yourself and the song. But even then, if you listen hard, you will listen to the two separate voices, one from yourself and one from the speakers. They won't blur, they won't unify, no matter how good a singer you are.
Now, when you are in the exalted state of being, and you have succumbed to the temptation of singing the same song aloud, mirth begins to rise with your voice, and you are singing the song, note for note. But then, there is a difference. Concentrate really hard. Really really hard. Try to distinguish the two sources of sound. You cannot. It is then that you feel you are in front of the throng of a million, performing with the precision of a master. There is no speaker. There is no song playing. It is just you singing. It is then..that the Duality of Music..ends! :)
Try making sense of this in sanity! :)