What is music?

So, last night I was showing Daniel "Threnody of the Victims of Hiroshima" by Penderecki. It's possibly one of the most disturbing pieces I've ever heard, but it also might be one of the best for the very reason that it managed to move me that much. One thing I've never been able to decide is whether or not that sound in the beginning is people screaming or if it's the bomb dropping thousands and thousands of feet in the air before finally slamming into the city. I tend to think it's the first way...but I've been wrong before.

Anyway here it is. Warning: it starts out very loud.

Of course when I showed that to Daniel, his immediate response was "I am prepared to argue that is not music." And like any other time we disagree strongly on something, a debate began. We started discussing what precisely defined music. Is the piece above really not music? What about John Cage's "4'33"? Does that count? Is Threnody belong in the same group as say Shostakovich's Symphony No. 5, 2nd mvt? How would you define music?

Side note: I adore Symphony No. 5. Supposedly it was written as his way of protesting the Soviet regime but that is the subject of much debate.