gistfile1.m

        NSArray* inputURLs = @[[NSURL fileURLWithPath:@"/Users/jon/Desktop/welcome.caf"], [NSURL fileURLWithPath:@"/Users/jon/Desktop/welcome.caf"]];
        NSError* error=nil;
        AVMutableComposition* composition = [AVMutableComposition composition];
        for(NSURL* url in inputURLs) {
            AVMutableCompositionTrack* track = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
            AVURLAsset* asset = [AVURLAsset assetWithURL:url];
            NSArray* assetTracks = [asset tracksWithMediaType:AVMediaTypeAudio];
            AVAssetTrack* assetTrack = assetTracks[0];
            [track insertTimeRange:assetTrack.timeRange ofTrack:assetTrack atTime:composition.duration error:&error];
        }
        
        AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetPassthrough];
        exporter.outputURL = [NSURL fileURLWithPath:@"/Users/jon/Desktop/output.caf"];
        exporter.outputFileType = AVFileTypeCoreAudioFormat;
        [exporter exportAsynchronouslyWithCompletionHandler:^{
            NSLog(@"Finished! %@ %@", exporter, exporter.error);
        }];