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.

spec/controllers/sounds_controller_spec.rb

@@ -0,0 +1,25 @@
+require 'spec_helper'
+
+describe SoundsController do
+  before do
+    controller.current_user = Factory.create(:user)
+  end
+
+  describe 'GET show' do
+    let(:sound) { Factory.create(:sound) }
+
+    def do_show
+      get :show, :id => sound.id
+    end
+
+    it 'sends the sound file as the response body' do
+      do_show
+      response.body.should == File.read(sound.path)
+    end
+
+    it 'sets the sound file mime-type as the response content-type' do
+      do_show
+      response.content_type.should == sound.mime_type
+    end
+  end
+end