Phantom of the orchestra

I often use (mostly classical) music to counteract boring background noise, using my portable CD player or the new MP3 player on the shuttle to Phoenix, and the laptop at work. I don't play it terribly loud, and I'm not usually concentrating on it. I've noticed that it sometimes takes me a while to register when the music stops; it's as if, when the earbuds are in place, my brain tries very hard to spin faint music out of random sounds, though what it gets is more like a distant orchestra tuning up than an actual performance.