Well try something like this
AVMutableAudioMix *exportAudioMix = [AVMutableAudioMix audioMix];
AVAssetTrack *assetTrack = [[asset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0];
AVMutableAudioMixInputParameters *exportAudioMixInputParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:assetTrack];
exportAudioMixInputParameters.trackID = [[[asset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0] trackID];
NSMutableArray* inputParameters = [NSMutableArray arrayWithCapacity:1];
CMTime startFadeInTime = start;
CMTime endFadeInTime = CMTimeMake((startTime+2)*100, 100);
CMTime startFadeOutTime = CMTimeMake((time-2)*100, 100);
CMTime endFadeOutTime = CMTimeMake(time*100, 100);
CMTimeRange fadeInTimeRange = CMTimeRangeFromTimeToTime(startFadeInTime, endFadeInTime);
CMTimeRange fadeOutTimeRange = CMTimeRangeFromTimeToTime(startFadeOutTime, endFadeOutTime);
[exportAudioMixInputParameters setVolumeRampFromStartVolume:0.0 toEndVolume:1.0 timeRange:fadeInTimeRange];
[exportAudioMixInputParameters setVolumeRampFromStartVolume:1.0 toEndVolume:0.0 timeRange:fadeOutTimeRange];
[inputParameters insertObject:exportAudioMixInputParameters atIndex:0];
Hope it will help. Happy coding.