title: AVAssetTrack: Audio tracks don't have a timeRange?
body: <p>With the following, and an MP3 file:</p>
  
  <pre><code>AVURLAsset* audio = [[AVURLAsset alloc]initWithURL:audioUrl options:nil];
  AVAssetTrack* audioTrack = [[audio tracksWithMediaType:AVMediaTypeAudio] 
      objectAtIndex:0];
  CMTime audioDuration = audio.duration;
  
  NSLog(@"audioDuration: %qi/%i; audioTrackDuration: %qi/%i", 
        audioDuration.value, audioDuration.timescale, 
        audioTrack.timeRange.duration.value,
        audioTrack.timeRange.duration.timescale);
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>audioDuration: 1513728/44100; audioTrackDuration: 0/1
  </code></pre>
  
  <p>Note that the track itself is fine when I make use of it later. I thought it might be an asynchronous key-value coding issue, but this <code>NSParameterAssert</code> seems to pass:</p>
  
  <pre><code>AVKeyValueStatus trStatus = [audioTrack statusOfValueForKey:@"timeRange" 
      error:&amp;theError];
  NSParameterAssert(trStatus == AVKeyValueStatusLoaded);
  </code></pre>
  
  <p>What's the deal? Is this just a quirk of the format?</p>