title: AVMutableCompositionTrack - insertTimeRange - insertEmptyTimeRange issue
body: <p>I have a strange problem: I want to generate a new sound file out of two soundfiles and silence.</p>
  
  <p>sound1: 2 seconds long
  +
  silence: 2 seconds silence
  +
  sound2: 2 seconds long</p>
  
  <p>When I try the code below, I get a 6 seconds long soundfile with all the parts, but in a different order! The order is: sound1, sound2, silence</p>
  
  <p>I am not able to put this silence in the middle of this composition (also not at the beginning). Is this a typical behavior or do I something wrong?</p>
  
  <p>Here is the code for putting the segments together:</p>
  
  <pre><code>[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [audio1 duration])  ofTrack:clipAudioTrack1 atTime:kCMTimeZero error:nil];
  [compositionAudioTrack insertEmptyTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(2, 1))];
  [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [audio2 duration])  ofTrack:clipAudioTrack2 atTime:CMTimeMake(4, 1) error:nil];
  </code></pre>
  
  <p>Perhaps someone has an idea? Thank you in advance!</p>
  
  <p>By the way: following code without the insertEmptyTimeRange-line doesn't work either, it just generates 4 seconds of sound and sound2 slides to the end of sound1:</p>
  
  <pre><code>    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [audio1 duration])  ofTrack:clipAudioTrack1 atTime:kCMTimeZero error:nil];
  [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [audio2 duration])  ofTrack:clipAudioTrack2 atTime:CMTimeMake(4, 1) error:nil];
  </code></pre>
  
  <p>It seems that it is not allowed that there is "nothing" between segments!?</p>