Audio video merge with main video sound

问题

I can merge a video with another audio nicely. But I also need main video sound in final output video. That means I want to set main video sound with low volume. How can I do this?

 -(void)mergeAndSave
{
//Create AVMutableComposition Object which will hold our multiple AVMutableCompositionTrack or we can say it will hold our video and audio files.
AVMutableComposition* mixComposition = [AVMutableComposition composition];

//Now first load your audio file using AVURLAsset. Make sure you give the correct path of your videos.
NSURL *audio_url = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"Asteroid_Sound" ofType:@"mp3"]];
AVURLAsset  *audioAsset = [[AVURLAsset alloc]initWithURL:audio_url options:nil];
CMTimeRange audio_timeRange = CMTimeRangeMake(kCMTimeZero, audioAsset.duration);

//Now we are creating the first AVMutableCompositionTrack containing our audio and add it to our AVMutableComposition object.
AVMutableCompositionTrack *b_compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];

//Now we will load video file.
NSURL *video_url = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"Asteroid_Video" ofType:@"m4v"]];
AVURLAsset  *videoAsset = [[AVURLAsset alloc]initWithURL:video_url options:nil];
CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,audioAsset.duration);

//Now we are creating the second AVMutableCompositionTrack containing our video and add it to our AVMutableComposition object.
AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[a_compositionVideoTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

//decide the path where you want to store the final video created with audio and video merge.
NSArray *dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *docsDir = [dirPaths objectAtIndex:0];
NSString *outputFilePath = [docsDir stringByAppendingPathComponent:[NSString stringWithFormat:@"FinalVideo.mov"]];
NSURL *outputFileUrl = [NSURL fileURLWithPath:outputFilePath];
if ([[NSFileManager defaultManager] fileExistsAtPath:outputFilePath])
    [[NSFileManager defaultManager] removeItemAtPath:outputFilePath error:nil];

//Now create an AVAssetExportSession object that will save your final video at specified path.
AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
_assetExport.outputFileType = @"com.apple.quicktime-movie";
_assetExport.outputURL = outputFileUrl;

[_assetExport exportAsynchronouslyWithCompletionHandler:
 ^(void ) {

     dispatch_async(dispatch_get_main_queue(), ^{
         [self exportDidFinish:_assetExport];
     });
 }
 ];
}

This is my code how I merge a video with audio.

回答1:

You can have multiple Audio tracks in a composition, you can create another track that would have the main video's audio:

AVMutableCompositionTrack *mainVideoAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[mainVideoAudioTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];

In order to reduce the volume you would need to create use AVAudioMix:

AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];

//Instruction for main video audio track
AVMutableAudioMixInputParameters *mainAudioMixParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:mainVideoAudioTrack];
[mainAudioMixParams setVolume:0.25 atTime:kCMTimeZero];

//Instruction for background audio track
AVMutableAudioMixInputParameters *b_audioMixParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:b_compositionAudioTrack];
[b_audioMixParams setVolume:1 atTime:kCMTimeZero];

audioMix.inputParameters = @[b_audioMixParams, mainAudioMixParams];

Then when you are exporting you need to add the audio mix to the export session as so:

_assetExport.audioMix = audioMix;

来源：https://stackoverflow.com/questions/41987316/audio-video-merge-with-main-video-sound