正如可以用AVVideoComposition實現(xiàn)視頻的混合一樣，也可以用AVAudioMix實現(xiàn)音頻的混合，用法也跟AVVideoComposition很相似，
Demo見https://github.com/Phelthas/TEST_XMLCommon

首先創(chuàng)建一個AVMutableComposition，將需要混音的視頻或者音頻軌道添加進去

    AVAssetTrack *videoTrack = [videoAsset tracksWithMediaType:AVMediaTypeVideo].firstObject;
    AVAssetTrack *audioTrack1 = [videoAsset tracksWithMediaType:AVMediaTypeAudio].firstObject;
    AVAssetTrack *audioTrack2 = [audioAsset tracksWithMediaType:AVMediaTypeAudio].firstObject;
    AVMutableComposition *composition = [AVMutableComposition composition];
    AVMutableCompositionTrack *videoCompositionTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [videoCompositionTrack insertTimeRange:videoTimeRange ofTrack:videoTrack atTime:kCMTimeZero error:nil];
    AVMutableCompositionTrack *audioCompositionTrack1 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [audioCompositionTrack1 insertTimeRange:videoTimeRange ofTrack:audioTrack1 atTime:kCMTimeZero error:nil];
    AVMutableCompositionTrack *audioCompositionTrack2 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [audioCompositionTrack2 insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration) ofTrack:audioTrack2 atTime:kCMTimeZero error:nil];
    self.composition = composition;

然后創(chuàng)建AVMutableAudioMix，創(chuàng)建對應(yīng)音軌的AVMutableAudioMixInputParameters并設(shè)置音量，賦值給AVMutableAudioMix的inputParameters即可。

注意這里最好用添加到composition中的AVMutableCompositionTrack來初始化AVMutableAudioMixInputParameters，否則還需要手動設(shè)置trackID。（每個track的trackID必須不一樣）

    AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
    // 注意：這里必須要用audioCompositionTrack1而不能用audioTrack1，否則會因為trackId不一致而不生效
    AVMutableAudioMixInputParameters *inputParameters1 = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:audioCompositionTrack1];
    [inputParameters1 setVolume:1 atTime:kCMTimeZero];
    AVMutableAudioMixInputParameters *inputParameters2 = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:audioCompositionTrack2];
    [inputParameters2 setVolume:0.5 atTime:kCMTimeZero];
    audioMix.inputParameters = @[inputParameters1, inputParameters2];
    self.audioMix = audioMix;

最后將audioMix賦值給playerItem即可

    AVPlayerItem *playerItem = [AVPlayerItem playerItemWithAsset:composition];
    playerItem.audioMix = audioMix;
    self.playerItem = playerItem;
    AVPlayer *player = [AVPlayer playerWithPlayerItem:playerItem];
    self.player = player;
    [player play];

完成~

如果需要實時修改混音的音量（比如用slider分別控制音軌的音量），需要每次都重新生成AVAudioMix才行，且里面的inputParameters也需要重新生成或者mutableCopy，直接修改原來的音量再賦值是不行的

    AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
    AVMutableAudioMixInputParameters *parameters1 = [[self.audioMix.inputParameters objectAtIndex:0] mutableCopy];
    [parameters1 setVolume:self.slider1.value atTime:kCMTimeZero];
    AVMutableAudioMixInputParameters *parameters2 = [[self.audioMix.inputParameters objectAtIndex:1] mutableCopy];
    [parameters2 setVolume:self.slider2.value atTime:kCMTimeZero];
    audioMix.inputParameters = @[parameters1, parameters2];
    self.playerItem.audioMix = audioMix;

導出時只需要將AudioMix賦值給exportSession即可，其他完全一樣
注意：導出后多個音軌會合并成一個

    NSString *userCacheDirectory = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)[0];
    NSString *outputURLString = [NSString stringWithFormat:@"%@/audioMixOutput.mp4", userCacheDirectory];
    NSLog(outputURLString);
    [kLXMFileManager removeItemAtPath:outputURLString error:nil];
    AVAssetExportSession *exportSession = [AVAssetExportSession exportSessionWithAsset:self.composition presetName:AVAssetExportPresetHighestQuality];
    exportSession.outputFileType = AVFileTypeMPEG4;
    exportSession.outputURL = [NSURL fileURLWithPath:outputURLString];
    exportSession.shouldOptimizeForNetworkUse = YES;
    exportSession.audioMix = self.audioMix;
    [exportSession exportAsynchronouslyWithCompletionHandler:^{
        if (exportSession.status == AVAssetExportSessionStatusCompleted) {
            NSLog(@"export success");
        } else {
            NSLog(@"export failed: %@", exportSession.error);
        }
    }];

完整Demo