Merging Videos Together with AVMutableComposition Causes No Audio - ios

Question

I have an NSArray containing a list of video NSURL's and I want to merge them together to make one long compilation. The problem is, when I use the code below the videos merge but there is no audio.
- (IBAction)buildVideo {
// 1 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.
AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
AVMutableCompositionTrack *track = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
int i = 0;
for (id object in movieArray) {
AVAsset *asset = [AVAsset assetWithURL:object];
if(i == 0){
[track insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration)
ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
}else {
[track insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration)
ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:[mixComposition duration] error:nil];
}
i = i + 1;
}
// 4 - Get path
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent:
[NSString stringWithFormat:#"mergeVideo-%d.mov",arc4random() % 1000]];
NSURL *url = [NSURL fileURLWithPath:myPathDocs];
// 5 - Create exporter
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
presetName:AVAssetExportPresetHighestQuality];
exporter.outputURL=url;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.shouldOptimizeForNetworkUse = YES;
[exporter exportAsynchronouslyWithCompletionHandler:^{
dispatch_async(dispatch_get_main_queue(), ^{
[self exportDidFinish:exporter];
});
}];
}

Answer 1

The reason you're not getting audio is that you're not adding the audio track. You need to create an additional AVMutableCompositionTrack with a type of AVMediaTypeAudio:
AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
preferredTrackID:kCMPersistentTrackID_Invalid];
And insert the time range for the source audio and video tracks into both composition tracks:
CMTime insertTime = kCMTimeZero;
for (id object in movieArray) {
AVAsset *asset = [AVAsset assetWithURL:object];
CMTimeRange timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration);
[videoTrack insertTimeRange:timeRange
ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
atTime:insertTime
error:nil];
[audioTrack insertTimeRange:timeRange
ofTrack:[[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]
atTime:insertTime
error:nil];
insertTime = CMTimeAdd(insertTime,asset.duration);
}