第一种可网络可本地，可调整音量，范围等定制化还比较高

1.基本类介绍
refer：https://docs.microsoft.com/en-us/dotnet/api/avfoundation.avcomposition?view=xamarin-ios-sdk-12

AVAsset 媒体信息
AVURLAsset 根据URL路径创建的媒体信息
AVAssetTrack 资源轨道，包括音频轨道和视频轨道
AVMutableAudioMixInputParameters音频操作参数
AVMutableComposition 继承自AVComposition，用于从现有Asset创建新合成的可变对象。包含多个轨道的媒体信息，可以添加、删除轨道
AVAssetExportSession 导出

具体实现如下：

#pragma mark - 音频与音频的合并
+ (void)mixOriginalAudio:(NSURL *)originalAudioPath
     originalAudioVolume:(float)originalAudioVolume
             bgAudioPath:(NSURL *)bgAudioPath
           bgAudioVolume:(float)bgAudioVolume
          outPutFileName:(NSString *)fileName
         completionBlock:(CompletionBlock)completionBlock
{
    if (originalAudioPath == nil) {
        return;
    }
    if (bgAudioPath == nil) {
        return;
    }
    if (originalAudioVolume > 1.0) {
        originalAudioVolume = 1.0f;
    }
    if (originalAudioVolume < 0) {
        originalAudioVolume = 0.0f;
    }
    if (bgAudioVolume > 1.0) {
        bgAudioVolume = 1.0f;
    }
    if (bgAudioVolume < 0) {
        bgAudioVolume = 0.0f;
    }

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        AVURLAsset *originalAudioAsset = [AVURLAsset assetWithURL:originalAudioPath];
        AVURLAsset *bgAudioAsset = [AVURLAsset assetWithURL:bgAudioPath];

        AVMutableComposition *compostion = [AVMutableComposition composition];

        AVMutableCompositionTrack *originalAudio = [compostion addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];
        [originalAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, originalAudioAsset.duration) ofTrack:[originalAudioAsset tracksWithMediaType:AVMediaTypeAudio].firstObject atTime:kCMTimeZero error:nil];

        AVMutableCompositionTrack *bgAudio = [compostion addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];

        [bgAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, originalAudioAsset.duration) ofTrack:[bgAudioAsset tracksWithMediaType:AVMediaTypeAudio].firstObject atTime:kCMTimeZero error:nil];
        //播放速率会受影响
        NSUInteger originalAudioAssetTotalSeconds = CMTimeGetSeconds(originalAudioAsset.duration);
        NSUInteger bgAudioAssetTotalSeconds = CMTimeGetSeconds(bgAudioAsset.duration);
        if (originalAudioAssetTotalSeconds>bgAudioAssetTotalSeconds) {
            [bgAudio scaleTimeRange:CMTimeRangeMake(kCMTimeZero, bgAudioAsset.duration) toDuration:originalAudioAsset.duration];
        }
        /** 得到对应轨道中的音频声音信息，并更改 */
        AVMutableAudioMixInputParameters *originalAudioParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:originalAudio];
        [originalAudioParameters setVolume:originalAudioVolume atTime:kCMTimeZero];

        AVMutableAudioMixInputParameters *bgAudioParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:bgAudio];
        [bgAudioParameters setVolume:bgAudioVolume atTime:kCMTimeZero];

        /** 赋给对应的类 */
        AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
        audioMix.inputParameters = @[ originalAudioParameters, bgAudioParameters ];

        AVAssetExportSession *session = [[AVAssetExportSession alloc] initWithAsset:compostion presetName:AVAssetExportPresetAppleM4A];

        /** 设置输出路径 */
        NSURL *outputPath = [self exporterAudioPathWithFileName:fileName];

        session.audioMix = audioMix;
        session.outputURL = outputPath;
        session.outputFileType = AVFileTypeAppleM4A;
        session.shouldOptimizeForNetworkUse = YES;

        [session exportAsynchronouslyWithCompletionHandler:^{

            dispatch_async(dispatch_get_main_queue(), ^{

                switch ([session status]) {
                    case AVAssetExportSessionStatusFailed: {
                        NSLog(@"合成失败：%@", [[session error] description]);
                        completionBlock(NO, outputPath);
                    } break;

                    case AVAssetExportSessionStatusCancelled: {
                        completionBlock(NO, outputPath);
                    } break;

                    case AVAssetExportSessionStatusCompleted: {
                        completionBlock(YES, outputPath);

                    } break;

                    default: {
                        completionBlock(NO, outputPath);
                    } break;
                }

            });


        }];

    });
}

⚠️压缩时长

更改给定时间范围内所有轨道的时长。

- (void)scaleTimeRange:(CMTimeRange)timeRange
toDuration:(CMTime)duration;
参数timeRange:要缩放的合成轨道的时间范围。
参数duration:新的时长。

受缩放操作影响的每个轨道段将以等于其结果时间映射的source.duration/target.duration的速率呈现。

⚠️设置背景音乐时长和插入时间

   [bgAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, originalAudioAsset.duration) ofTrack:[bgAudioAsset tracksWithMediaType:AVMediaTypeAudio].firstObject atTime:kCMTimeZero error:nil];
因为我的需求是加入背景音乐给录音，所以将背景音乐从头插入kCMTimeZero 插入时长为录音的时长 originalAudioAsset.duration