/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
...

##</code><code>##

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