Works for me maybe Celestia could hear Luna sing from the towers they reside in are both being insight of the other thereby Celestia finding out of Lunas singing talent. Then again I had another ideal that would have Celestia finding out Lunas talent via a sad'ish song namely this one
http://www.youtube.com/watch?v=LDQG6hbceRM The song showing how insecure Luna is from her past actions and being exposed to a whole new set of cultural standards while having to hide her trueself to fit in with the modern cultural setup