c                             /c/
   aa                         /c/aa/
   bb                         /c/bb/
      aa1.mp3                 /c/bb/aa1.mp3
      aa2.mp3                 /c/bb/aa2.mp3
   cc                         /c/cc/
   a1.mp3                     /c/a1.mp3
   a2.mp3                     /c/a2.mp3