The definition of music
There is no single universally agreed-upon definition of music. We know it when we hear it, but what you might hear as music, I might hear as noise. Who's right? We both are. The entire high modernist movement of the twentieth century was devoted to finding out just far you can push the limits of the conventional definition of music. John Cage considered all of his work to be music. A normal person would likely consider just about none of it to be music. I consider it some of it to be music, mostly extremely annoying music, and some of it to be clever or not-so-clever conceptual art. There isn't an ultimate authority we can appeal to who will rule on whose opinion is the "right" one (though my NYU professors would disagree with me there.)
I read in my music psychology textbook how the Basongye people of the Congo hinge their definition of music on the performer's state of mind. By Basongye standards, if you're content, tranquil or thoughtful, then your singing counts as music. If you're angry or shouting, you aren't singing, you're just making noise. That's a much narrower definition than the one we use in America -- probably the Basongye wouldn't consider Nirvana or Metallica to be music, whereas most of us here would. An even narrower one: when Elwood Blues asks the lady at Bob's Country Bunker what kind of music they usually have, she replies, "Oh, we got both kinds, country and western."
[youtube=http://www.youtube.com/watch?v=bryvnZYiIaY]
Classically-minded people in western cultures think of melody as a defining feature of music. This idea is institutionalized in our copyright laws; you can copyright a melody, but not a chord progression or rhythmic groove. Since hip-hop frequently lacks melody, there are plenty of people who don't consider it to be music. Millions of hip-hop fans the world over (myself included) don't see melody as a requirement for a sound to be musical. Those of us who prefer the music of the African diaspora consider rhythm to be the defining characteristic of music. In fact, if you want to be technical, a pitch is really just a very fast rhythm, and harmony is really just very fast polyrhythm. I'm an active member of the Disquiet Junto, a sound art and music collective. My Junto contributions are almost always beat-driven, and even when they're ambient, I still put them together on a 4/4 grid. When I can't think of a way to make a Junto assignment sound like music, then I sit it out. There are sound artists, in the Junto and elsewhere, who consider all of their work to be music, simply by virtue of their intending it to be so. So I could sit here and think, "okay, the next ten seconds worth of noise that comes in through the window is a piece of music." This is dubious to me, but I'll freely admit that once you're into ambient and found sounds, the "music/not music" distinction becomes vague. Maybe music just doesn't have clearly defined boundaries at all.