可以传网络连接进行合成

/**
 *  合并两段音频
 * 
 *  @param url              主音乐路径
 *  @param accompanimentUrl 背景音乐路径
 *  @param success          成功后回调
 *  @param failure          失败后回调
 */
-(void)audio:(NSURL *)url AndAccompaniment:(NSURL *)accompanimentUrl isSuccess:(void (^)(NSString *outPath))success failure:(void (^)())failure{
    //创建AVAsset
    AVURLAsset *audioAsset = [AVURLAsset assetWithURL:url];
    AVURLAsset *accompanimentAsset = [AVURLAsset assetWithURL:accompanimentUrl];
    AVMutableComposition *compostion = [AVMutableComposition composition];
    AVMutableCompositionTrack *audio = [compostion addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];
    [audio insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration) ofTrack:[audioAsset tracksWithMediaType:AVMediaTypeAudio].firstObject atTime:kCMTimeZero error:nil];
    AVMutableCompositionTrack *accompaniment = [compostion addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];
    [accompaniment insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration) ofTrack:[accompanimentAsset tracksWithMediaType:AVMediaTypeAudio].firstObject atTime:kCMTimeZero error:nil];//背景音频长度，以主音频长度为准audioAsset.duration
    
    AVAssetExportSession *session = [[AVAssetExportSession alloc]initWithAsset:compostion presetName:AVAssetExportPresetAppleM4A];
    //输出路径
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
    NSString *outPutFilePath = [NSString stringWithFormat:@"%@/audio.m4a",[paths lastObject]];
    if ([[NSFileManager defaultManager] fileExistsAtPath:outPutFilePath])
    {
        [[NSFileManager defaultManager] removeItemAtPath:outPutFilePath error:nil];
    }
    session.outputURL = [NSURL fileURLWithPath:outPutFilePath];
    session.outputFileType = @"com.apple.m4a-audio";
    session.shouldOptimizeForNetworkUse = YES;
    [session exportAsynchronouslyWithCompletionHandler:^{
        if ([[NSFileManager defaultManager] fileExistsAtPath:outPutFilePath])
        {
            // 成功调用
         /*注意，此处如果处理UI需要回主线程*/
            dispatch_async(dispatch_get_main_queue(), ^{
                success(outPutFilePath);
            });
            
        }
        else
        {   //失败调用
            dispatch_async(dispatch_get_main_queue(), ^{
                failure();
            });
        }
    }];

}