scaleTimeRange has no effect on audio type AVMutableCompositionTrack

To be able to play slow mo with sound just set appropriate audioTimePitchAlgorithm for AVPlayerItem

playerItem.audioTimePitchAlgorithm = AVAudioTimePitchAlgorithmVarispeed;

ScaleTimeRange does not work for audio due to a bug in apple api (Bug ID : 14616144). However, time scaling audio could be done using resampling. If audio is sampled at 48 kHz, resample to 96 kHz and play the audio at 48 kHz this will take twice as long to play. Generally:

scaledSampleRate = (orignalSampleRate / playRate);
playRate = (originalSampleRate / scaledSampleRate);

The pitch will be lowered though, if this is not desired. The solution might be a third party api. If you can extract the audio to a separate player i guess you could use AVAudioPlayer, where you can set the play-rate:

player = [[AVAudioPlayer alloc] initWithContentsOfURL:
                  [NSURL fileURLWithPath:path] error:&err];
        player.volume = 0.4f;
        player.enableRate=YES;
        [player prepareToPlay];
        [player setNumberOfLoops:0];
        player.rate=2.0f;
        [player play];