AVMutableCompositionTrack setVolume not working

I'm having problem to set the audio volume when mixing a recorded video and an audio from resource.

Here is my code:

AVMutableComposition *composition = [[AVMutableComposition alloc] init];
NSString *resourcePath = [[NSBundle mainBundle] pathForResource:@"give-it-away" ofType:@"mp3"];
AVURLAsset *audioAsset = [[AVURLAsset alloc] initWithURL:[NSURL fileURLWithPath:resourcePath] options:[NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES],AVURLAssetPreferPreciseDurationAndTimingKey, nil]];
AVMutableCompositionTrack *videoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *audioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration) ofTrack:[videoAsset.tracks objectAtIndex:0] atTime:kCMTimeZero error:nil];
[audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioTime) ofTrack:[audioAsset.tracks objectAtIndex:0] atTime:kCMTimeZero error:&videoError];

AVMutableAudioMixInputParameters *audioInputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:audioTrack] ;
[audioInputParams setVolume:0.3 atTime:kCMTimeZero];
[audioInputParams setTrackID:audioTrack.trackID];
AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
audioMix.inputParameters = [NSArray arrayWithObject:audioInputParams];

AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetPassthrough];

exportSession.outputURL = [NSURL fileURLWithPath:finalVideoWithAudioPath];
exportSession.outputFileType = AVFileTypeQuickTimeMovie;            
exportSession.audioMix = audioMix;            
[exportSession exportAsynchronouslyWithCompletionHandler:^{
      switch (exportSession.status)
      {
          case AVAssetExportSessionStatusFailed:
          {
               [self performSelectorOnMainThread:@selector(doPostExportFailed) withObject:nil waitUntilDone:NO];
               break;
          }
          case AVAssetExportSessionStatusCompleted:
          {
               [self performSelectorOnMainThread:@selector(doPostExportSuccess) withObject:nil waitUntilDone:YES];
               break;
          }
      };
}];

The export completes successfully, but the audio volume does not change. What am I doing wrong?

Thanks