How to concatenate 2 or 3 audio files in iOS?

Question

I am newcomer in Objective-C and have experience only 5 months in iPhone development.

What I need:
I need to concatenate 2 or more audio files into one, and export result as aiff, mp3, caf or m4a format.

For example:
First audio file containing "You need", second "download" and third "document".
Every audio part depends on actions from user.

I spent 2 days without luck. That place is my last frontier.

I will very appreciate for a piece of code.

Answer 1

Code below can be used to merge audio files.

Input files: Ids of the files to be supplied in array audioIds. Eg. audio1.mp3, audio2.mp3 … audioN.mp3 to be available in documents folder Output file: combined.m4a

     - (BOOL) combineVoices {

       NSError *error = nil;
       BOOL ok = NO;


         NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,    NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0]; 


       CMTime nextClipStartTime = kCMTimeZero;
        //Create AVMutableComposition Object.This object will hold our multiple AVMutableCompositionTrack.
        AVMutableComposition *composition = [[AVMutableComposition alloc] init];

     AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];

     for (int i = 0; i< [self.audioIds count]; i++) {
        int key = [[self.audioIds objectAtIndex:i] intValue];
        NSString *audioFileName = [NSString stringWithFormat:@"audio%d", key];

        //Build the filename with path
        NSString *soundOne = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.mp3", audioFileName]];
        //NSLog(@"voice file - %@",soundOne);

        NSURL *url = [NSURL fileURLWithPath:soundOne];    
        AVAsset *avAsset = [AVURLAsset URLAssetWithURL:url options:nil];
        NSArray *tracks = [avAsset tracksWithMediaType:AVMediaTypeAudio];
        if ([tracks count] == 0) 
            return NO;
        CMTimeRange timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, [avAsset duration]);
        AVAssetTrack *clipAudioTrack = [[avAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        ok = [compositionAudioTrack insertTimeRange:timeRangeInAsset  ofTrack:clipAudioTrack atTime:nextClipStartTime error:&error];
        if (!ok) {
            NSLog(@"Current Video Track Error: %@",error);
        }
        nextClipStartTime = CMTimeAdd(nextClipStartTime, timeRangeInAsset.duration);
    }

    // create the export session
    // no need for a retain here, the session will be retained by the
    // completion handler since it is referenced there
    AVAssetExportSession *exportSession = [AVAssetExportSession
                                           exportSessionWithAsset:composition
                                           presetName:AVAssetExportPresetAppleM4A];
    if (nil == exportSession) return NO;

    NSString *soundOneNew = [documentsDirectory stringByAppendingPathComponent:@"combined.m4a"];
    //NSLog(@"Output file path - %@",soundOneNew);

    // configure export session  output with all our parameters
    exportSession.outputURL = [NSURL fileURLWithPath:soundOneNew]; // output path
    exportSession.outputFileType = AVFileTypeAppleM4A; // output file type

    // perform the export
    [exportSession exportAsynchronouslyWithCompletionHandler:^{

        if (AVAssetExportSessionStatusCompleted == exportSession.status) {
            NSLog(@"AVAssetExportSessionStatusCompleted");
        } else if (AVAssetExportSessionStatusFailed == exportSession.status) {
            // a failure may happen because of an event out of your control
            // for example, an interruption like a phone call comming in
            // make sure and handle this case appropriately
            NSLog(@"AVAssetExportSessionStatusFailed");
        } else {
            NSLog(@"Export Session Status: %d", exportSession.status);
        }
    }];

    return YES;
}