iOS AVFoundation Export Session is missing audio

Question

I'm am using the iOS AVFoundation framework and I am able to successfully merge video tracks, with image overlays, and text overlays. However, my output file doesn't keep the audio intact from my original source video.

How can I make sure that the audio source from one of my videos stays with the new video I create?

EDIT

*Use this code to have a good example of how to accomplish this creating a video (with original audio). It was not obvious to me that I need to include the audio track seperatly when processing a video with AVFoundation. Hope this helps somebody else.

    AVAssetTrack *videoTrack = nil;
    AVAssetTrack *audioTrack = nil;
    CMTime insertionPoint = kCMTimeZero;

    if([[url tracksWithMediaType:AVMediaTypeVideo] count] != 0) {
        videoTrack = [url tracksWithMediaType:AVMediaTypeVideo][0];
    }

    if([[url tracksWithMediaType:AVMediaTypeAudio] count] != 0) {
        audioTrack = [url tracksWithMediaType:AVMediaTypeAudio][0];
    }

    // Insert the video and audio tracks from AVAsset
    if (videoTrack != nil) {
        AVMutableCompositionTrack *compositionVideoTrack = [videoComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [url duration]) ofTrack:videoTrack atTime:insertionPoint error:&error];
    }
    if (audioTrack != nil) {
        AVMutableCompositionTrack *compositionAudioTrack = [videoComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
        [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [url duration]) ofTrack:audioTrack atTime:insertionPoint error:&error];
    }

Answer 1

Here is the complete code which solved this, it has two videos whcih are combined with their audios:-

AVURLAsset* video1 = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:path1] options:nil];

AVURLAsset* video2 = [[AVURLAsset alloc]initWithURL:[NSURL fileURLWithPath:path2] options:nil];

if (video1 !=nil && video2!=nil) {

    // 1 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.
    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
    // 2 - Video track

    AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    AVMutableCompositionTrack *firstTrackAudio = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];

    [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, video1.duration)
                        ofTrack:[[video1 tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];
    [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, video2.duration)
                        ofTrack:[[video2 tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:video1.duration error:nil];

// it has an audio track

    if ([[video1 tracksWithMediaType:AVMediaTypeAudio] count] > 0)
    {
        AVAssetTrack *clipAudioTrack = [[video1 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        [firstTrackAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, video1.duration) ofTrack:clipAudioTrack atTime:kCMTimeZero error:nil];
    }

// it has an audio track

    if ([[video2 tracksWithMediaType:AVMediaTypeAudio] count] > 0)
    {
        AVAssetTrack *clipAudioTrack = [[video2 tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        [firstTrackAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, video2.duration) ofTrack:clipAudioTrack atTime:video1.duration error:nil];
    }

// export session

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetHighestQuality];

    //Creates the path to export to  - Saving to temporary directory
    NSString* filename = [NSString stringWithFormat:@"Video_%d.mov",arc4random() % 1000];
    NSString* path = [NSTemporaryDirectory() stringByAppendingPathComponent:filename];

    //Checks if there is already a file at the output URL.  
    if ([[NSFileManager defaultManager] fileExistsAtPath:path])
    {
        NSLog(@"Removing item at path: %@", path);
        [[NSFileManager defaultManager] removeItemAtPath:path error:nil];
    }

    exporter.outputURL = [NSURL fileURLWithPath:path];
    //Set the output file type
    exporter.outputFileType = AVFileTypeQuickTimeMovie;


    path3=path;
    [arr_StoredDocumentoryUrls addObject:path3];

    //Exports!
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        switch (exporter.status) {
            case AVAssetExportSessionStatusCompleted:{
                NSLog(@"Export Complete");

                break;
            }
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Export Error: %@", [exporter.error description]);
                break;
            case AVAssetExportSessionStatusCancelled:
                NSLog(@"Export Cancelled");
                break;
            default:
                break;
        }
    }];

}