Save track files in new Sound model:

* Consolidate migrations
* Add Sound model. Each sound can belong to a track and contains
  informations about one sound file.

app/controllers/sounds_controller.rb

@@ -0,0 +1,6 @@
+class SoundsController < ApplicationController
+  def show
+    sound = Sound.find params[:id]
+    send_file sound.path, :type => sound.mime_type
+  end
+end