- (void) setUpAndAddAudioAtPath:(NSURL*)assetURL toComposition:(AVMutableComposition *)composition
{
    AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

    AVMutableCompositionTrack *track = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    AVAssetTrack *sourceAudioTrack = [[songAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

    NSError *error = nil;
    BOOL ok = NO;

    CMTime startTime = CMTimeMakeWithSeconds(0,1);
    CMTime trackDuration = songAsset.duration;
    //CMTime longestTime = CMTimeMake(848896,44100); //(19.24 seconds)
    CMTimeRange tRange = CMTimeRangeMake(startTime,trackDuration);

    //Set Volume
    AVMutableAudioMixInputParameters *trackMix = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:track];
    [trackMix setVolume:0.8f atTime:startTime];
    [self.audioMixParams addObject:trackMix];

    //Insert audio into track
    ok = [track insertTimeRange:tRange ofTrack:sourceAudioTrack atTime:CMTimeMake(0,44100) error:&error];
}


- (IBAction)saveRecording
{
     AVMutableComposition *composition = [AVMutableComposition composition];
        audioMixParams = [[NSMutableArray alloc] initWithObjects:nil];

    //IMPLEMENT FOLLOWING CODE WHEN WANT TO MERGE ANOTHER AUdio FILE
    //Add Audio Tracks to Composition
    NSString *URLPath1 = [[NSBundle mainBundle] pathForResource:@"mysound" ofType:@"mp3"];
    NSString *URLPath2 = [[NSBundle mainBundle] pathForResource:@"mysound2" ofType:@"mp3"];
    NSURL *assetURL1 = [NSURL fileURLWithPath:URLPath1];
    [self setUpAndAddAudioAtPath:assetURL1   toComposition:composition];

    NSURL *assetURL2 = [NSURL fileURLWithPath:URLPath2];
    [self setUpAndAddAudioAtPath:assetURL2   toComposition:composition];

    AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
    audioMix.inputParameters = [NSArray arrayWithArray:audioMixParams];

    //If you need to query what formats you can export to,here's a way to find out
    NSLog (@"compatible presets for songAsset: %@",[AVAssetExportSession exportPresetsCompatibleWithAsset:composition]);

    AVAssetExportSession *exporter = [[AVAssetExportSession alloc]
                                      initWithAsset: composition
                                      presetName: AVAssetExportPresetAppleM4A];
    exporter.audioMix = audioMix;
    exporter.outputFileType = @"com.apple.m4a-audio";
    NSURL *exportURL = [NSURL fileURLWithPath:exportFile];
    exporter.outputURL = exportURL;

// do the export
        [exporter exportAsynchronouslyWithCompletionHandler:^{
            int exportStatus = exporter.status;
            NSError *exportError = exporter.error;

            switch (exportStatus) {
                case AVAssetExportSessionStatusFailed:

                    break;

                case AVAssetExportSessionStatusCompleted: NSLog (@"AVAssetExportSessionStatusCompleted"); 
                    break;
                case AVAssetExportSessionStatusUnknown: NSLog (@"AVAssetExportSessionStatusUnknown"); break;
                case AVAssetExportSessionStatusExporting: NSLog (@"AVAssetExportSessionStatusExporting"); break;
                case AVAssetExportSessionStatusCancelled: NSLog (@"AVAssetExportSessionStatusCancelled"); break;
                case AVAssetExportSessionStatusWaiting: NSLog (@"AVAssetExportSessionStatusWaiting"); break;
                default:  NSLog (@"didn't get export status"); break;
            }
        }];
 }

请注意我不久前做过这个,你可能需要一点点工作才能使它工作.但它确实在某一点上起作用.如果您遇到问题,请告诉我.