Actually, I think appreciation and enjoyment are related but not symonyms. Enjoyment is visceral and emotional, it denotes the sheer pleasure of the experience. Appreciation implies recognition of the elements of the music, why those particular elements were chosen, how they might have been different, etc., as well as the extra enjoyment that comes about as a result of that appreciation. Not that I'm trying to say that's what everybody means by the terms, but that's how I think of them and how I've heard some other people talk about them.