title: AVMutableAudioMixInputParameters setVolume atTime not working only with kCMTimeZero
body: <p>Im am developing an iOS app in Xcode. I am still new to programming. 
  I am trying to set the volume of an audiotrack while playing it with an <code>AVPlayer</code>. It is working great if a set the time at <code>kCMTimeZero</code> but I want it to set the volume one second after the button was pressed.</p>
  
  <p><strong><em>Not</em> working</strong></p>
  
  <pre><code>- (IBAction)maxVolButtonPressed:(id)sender {
      [audioInputParams1 setVolume:1 atTime:time1];
      [self applyAudioMix];
  }
  </code></pre>
  
  <p><strong>Working</strong></p>
  
  <pre><code>- (IBAction)minVolButtonPressed:(id)sender {
      [audioInputParams1 setVolume:0 atTime:kCMTimeZero];
      [self applyAudioMix];
  }
  </code></pre>
  
  <p>What should I write after <code>atTime</code> if I want a one second delay?</p>
  
  <p>ANSWER: 
  Ok so I figured it out. You have to add the time where the current item is at the moment. I use setVolumeRampFromStartVolume with a very little time interval instead of setVolume. setVolume fades to the given volume for some reason I haven't figured it out why. 
  It works for me like this:</p>
  
  <pre><code>CMTime time1 = CMTimeMake(1000, 1000); //2s
  CMTime time2 = CMTimeMake(1001, 1000); //3s
  CMTime timeCurrent = [player currentTime];
  CMTime time1Added = CMTimeAdd(timeCurrent, time1);
  CMTime time2Added = CMTimeAdd(timeCurrent, time2);
  CMTimeRange fadeInTimeRange = CMTimeRangeFromTimeToTime(time1Added, time2Added);
  [audioInputParams1 setVolumeRampFromStartVolume:0 toEndVolume:1 timeRange:fadeInTimeRange];
  [self applyAudioMix];
  </code></pre>