/a/f1.mp3
/a/f2.mp3
/a/b/f1.mp3
/a/b/f2.mp3
/a/b/f3.mp3
/a/c/f1.mp3
/a/c/f2.mp3
/a/c/e/f5.mp3
/a/c/e/f6.mp3
/b/d/f4.mp3
...