Art being a "selective recreation of reality according to the artist's metaphysical value judgements", what exactly does music recreate, and how? I'm still quite far from a definite answer for that, but the best I've found was the view of Louis Torres and Michelle Kahmi, in their book "What Art Is".
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    The first thing to keep in mind is that feelings have identity. It might be hard to explain what a feeling is in words, yet we do know that the feeling of anger is considerably different from that of joy. We might even have "mixed feelings" about something, that we don't quite know how to value, but even that is a distinct feeling of its own. No one in their right mind says "I'm think I'm happy, but I might just be disgusted" - one does not feel like the other.
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    The second thing to keep in mind is that there seems to be a link between sound and feeling. From the fear brought about by the loud, sudden low-pitched noise of thunder, to the many contextual hints we feed eachother with the tone of our voices during a conversation, we instantly associate particular textures, intervals and rhythms to particular feelings.
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    Music then, seems to be the art that explores that link to portray feeling. A song is the portrayal of a specific feeling (or sequence of feelings), by means of selectively recreating aspects of sound that evoke them. Unlike a painting or a sculpture, we don't get a feeling because of what is portrayed, and how - the feeling itself is what is portrayed.
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    That gives us a pretty decent form-and-content standard with which to judge a work of music. What feeling did the artist attempt to portray, and how did he do it?