“Ah, music," he said, wiping his eyes. "A magic beyond all we do here!” - Albus Dumbledore, Harry Potter and The Philospher's Stone
So, Dumbledore is pretty darned fond of music.
We also know that, under Dumbledore, Hogwarts has been barely regulated (it's mentioned later on that Dumbledore basically has free reign to teach what he likes and hire what teachers he likes).
There's also a fair amount of extracurricular activities (quidditch, gobstones, chess etc.). These kids get up to some things.
We know that there are some popular and successful musicians in the magical world (Celestina Warbeck and The Weird Sisters are among them). Back out in the real world young people are very likely to use their teenage years trying to emulate popular musicians.
There also seems to be (on the whole) very little poverty among Hogwarts students. The Weasleys are always a little short, but just about everyone else can afford a heap of books each year, uniform, wand, broom and a pet with no trouble. So it wouldn't be too ridiculous to get an instrument tutor in for kids who wanted to play.
And yet, Hogwarts has:
- No instrument tutors
- No students who play an instrument in their own time.
- No band/choir/orchestra (Remember, the movies are not canon).
- Nobody even seems to listen to music. (There are magical radios, but we only see them outside of Hogwarts)
So why not? The headmaster loves it, the kids love some musical acts (even though they don't seem to listen to them), but no kids care?