How can music be sad?

Great question. Taking a different approach from Schopenhauer, the older idea of the music of the spheres is suggestive, especially when brought into the conversation that some musicologists are having about universal music. Does our music, and even the musical sounds animals make (whale song, for instance), echo something else, something that transcends the physical world? Do our notes and expressions serve as a kind of call-and-response to something? This gets into metaphysical concerns, but it does lead to considerations of why human cultures have so many different forms of music that yet have so many commonalities.

How about happy-sad music? That's how one student once described the clarinet in the Adagio of Mozart's Clarinet Concert. "Autumnal" etc., but with an underlying joyfulness. That's weird, and beautiful.  

The "music of the spheres" was probably meant as a mathematical idea, having to do with the ratios of planetary orbits. Plato thought that such massive objects moving rapidly through space must make a sound--but that sound would have been determined by "rational" intervals best (or most abstractly) described by mathematical relations. Even before Plato, Pythagoras noticed the mathematical relationships between octaves, etc. Much of our musical terminology (e.g., harmonic mean; chord progression; time signatures expressed in fractions; even pitch, measured in cycles per second) derive from the language of mathematics. In this context, Leibniz remarked that music was "an unconscious exercise in arithmetic in which the mind does not realize it is counting."

But one must be careful in supposing musical aesthetics to be "universal." Japanese music, Indonesian gamelan music--and, for that matter, ancient Greek music, so far as we've been able to reconstruct it--don't rely on the key relationships we take for granted in western music. The perception of "beauty" in music, and also mood, is a highly culturally specific thing, and depends on musical conventions that are not universal.

All these thoughtful responses have made me think about the question a bit more. Does our emotional reaction to music come from the same place as our reaction to the tone of someone's voice? Even when we can't see someone's face, as when we're talking to them on the phone, we can tell how they're feeling by the sound of their voice, or at least we think we can. 

Whether there's a definitive answer I'm glad the question managed to bring together Tolstoy, Shakespeare, David Allen Coe, Schopenhauer and Black Sabbath.

The GREAT thing about music is it can be a best friend to anyone in any stage of your life.

To snilf:

It's a good question to ponder, the universal nature of music, or its cultural specificity. The Bach phenomena in Japan is interesting in this regard. A 17th-century German Lutheran composer whose very cultural-specific music somehow becomes all the rage among late 20th/early 21st-century Japanese. How is that possible? The perceptions of beauty would seem to differ, yet the popularity of Bach's choral music in contemporary Japan begs some questions. The high suicide rates and despair evident in Japanese society, according to some interpreters, suggests a fertile ground for the sense of hope that animates Bach's music. Not just the lyrics of his orotorios, passions, or cantatas, either, but the music itself. It doesn't seem to matter that Bach's music is rooted in Early Modern western patterns, and that these are foreign to Japanese modes of thought. The response to the efforts of Masaaki Suzuki and his Bach Collegium Japan suggests that the bridges assumed by advocates of the music of the spheres are more than imaginary constructs rooted merely in ancient mathematical ideas. 

The conceptual origins of the music of the spheres provide a foundation that developed in different directions, mathematical, metaphysical, and musicological. Some current theories, even if speculative, remind us that the different languages of music transcend the local, and that our emotional response to sound waves requires a whole lot of wonder and curiosity. Count me in.