title: AVPlayer playback rate for AVComposition
body: <p>I am using an <code>AVPlayer</code> to handle playback of multiple assets by putting them all in an <code>AVMutableComposition</code>.  Everything works for normal playback, i.e. <code>[player play]</code>, but when I try something like <code>[player setRate:0.5]</code>, only one of the tracks in the composition slows down to half speed.  So I still have two tracks playing at the same time, but one is half speed and the other is full speed.  </p>
  
  <p>Does anyone have any idea how to get them both to slow down to the desired playback speed? Thanks</p>