最近帮朋友做个小功能，视频添加声音，可以修改添加音频开始时间、音量大小等，封装了个工具类，不多说，直接看代码吧。

XXWVideoEditor.h文件

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface XXWVideoEditor : NSObject

/**
 视频添加音频
 
 @param videoUrl 视频URL
 @param audioUrl 音频URL
 @param startTime 音频插入开始时间
 @param endTime 音频插入结束时间
 @param isOriginal 是否保留原声
 @param oriVolume 原声音量
 @param newVolume 插入的音频音量
 @param completionHandle 完成回调
 */
+ (void)addBackgroundMiusicWithVideoUrlStr:(NSURL *)videoUrl
                                  audioUrl:(NSURL *)audioUrl
                                     start:(CGFloat)startTime
                                       end:(CGFloat)endTime
                           isOriginalSound:(BOOL)isOriginal
                                 oriVolume:(float)oriVolume
                                 newVolume:(float)newVolume
                                completion:(void (^)(NSString *outPath,BOOL isSuccess))completionHandle;

@end

XXWVideoEditor.m文件

#import "XXWVideoEditor.h"

@implementation XXWVideoEditor

+ (void)addBackgroundMiusicWithVideoUrlStr:(NSURL *)videoUrl
                                  audioUrl:(NSURL *)audioUrl
                                     start:(CGFloat)startTime
                                       end:(CGFloat)endTime
                           isOriginalSound:(BOOL)isOriginal
                                 oriVolume:(float)oriVolume
                                 newVolume:(float)newVolume
                                completion:(void (^)(NSString *outPath,BOOL isSuccess))completionHandle
{
    //音频来源路径
    NSURL   *audio_inputFileUrl = audioUrl;
    
    //视频来源路径
    NSURL   *video_inputFileUrl = videoUrl;
    
    //导出路径
    NSString *outputFilePath = [XXWVideoEditor fileSavePath];
    NSURL   *outputFileUrl = [NSURL fileURLWithPath:outputFilePath];
    
    CMTime nextClipStartTime = kCMTimeZero;
    
    //创建可变的音频视频组合
    AVMutableComposition* mixComposition =[AVMutableComposition composition];
    
    //视频采集
    AVURLAsset* videoAsset =[[AVURLAsset alloc]initWithURL:video_inputFileUrl options:nil];
    CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);
    AVMutableCompositionTrack*a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                                    preferredTrackID:kCMPersistentTrackID_Invalid];
    [a_compositionVideoTrack insertTimeRange:video_timeRange
                                     ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
                                      atTime:nextClipStartTime
                                       error:nil];
    
    CMTime start = CMTimeMakeWithSeconds(startTime, videoAsset.duration.timescale);
    CMTime duration = CMTimeMakeWithSeconds(endTime - startTime,videoAsset.duration.timescale);
    CMTimeRange audio_timeRange = CMTimeRangeMake(start, duration);
    
    //创建最终混合的音频实例
    AVMutableAudioMix *audioMix = [AVMutableAudioMix audioMix];
    
    //新添加的音频
    AVURLAsset* audioAsset =[[AVURLAsset alloc]initWithURL:audio_inputFileUrl options:nil];
    AVMutableCompositionTrack *newAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                           preferredTrackID:kCMPersistentTrackID_Invalid];
    [newAudioTrack insertTimeRange:audio_timeRange
                           ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0]
                            atTime:start
                             error:nil];
    
    AVMutableAudioMixInputParameters *newAudioInputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:newAudioTrack] ;
    [newAudioInputParams setVolumeRampFromStartVolume:newVolume toEndVolume:.0f timeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)];
    [newAudioInputParams setTrackID:newAudioTrack.trackID];

    //视频文件原始音频通道
    if (isOriginal) {
        //视频声音采集(也可不执行这段代码不采集视频音轨，合并后的视频文件将没有视频原来的声音)
        AVMutableCompositionTrack *originVoiceTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
        CMTimeRange videoTimeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);
        [originVoiceTrack insertTimeRange:videoTimeRange ofTrack:([videoAsset tracksWithMediaType:AVMediaTypeAudio].count>0)?[videoAsset tracksWithMediaType:AVMediaTypeAudio].firstObject:nil atTime:kCMTimeZero error:nil];
        
        AVMutableAudioMixInputParameters *originAudioInputParams = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:originVoiceTrack] ;
        [originAudioInputParams setVolumeRampFromStartVolume:oriVolume toEndVolume:.0f timeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)];
        [originAudioInputParams setTrackID:originVoiceTrack.trackID];
        audioMix.inputParameters = [NSArray arrayWithObjects:newAudioInputParams,originAudioInputParams,nil];
    }else{
        audioMix.inputParameters = [NSArray arrayWithObject:newAudioInputParams];
    }
    
    //创建一个输出
    AVAssetExportSession *_assetExport =[[AVAssetExportSession alloc]initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
    _assetExport.outputFileType = AVFileTypeQuickTimeMovie;
    _assetExport.outputURL = outputFileUrl;
    _assetExport.shouldOptimizeForNetworkUse= YES;
    _assetExport.audioMix = audioMix;
    
    [_assetExport exportAsynchronouslyWithCompletionHandler:
     ^(void ) {
         switch ([_assetExport status]) {
             case AVAssetExportSessionStatusFailed: {
                 NSLog(@"合成失败：%@",[[_assetExport error] description]);
                 completionHandle(outputFilePath,NO);
             } break;
             case AVAssetExportSessionStatusCancelled: {
                 completionHandle(outputFilePath,NO);
             } break;
             case AVAssetExportSessionStatusCompleted: {
                 completionHandle(outputFilePath,YES);
             } break;
             default: {
                 completionHandle(outputFilePath,NO);
             } break;
         }
     }
     ];
}

+ (NSString *)fileSavePath
{
    NSString *documentsDirectory =[NSHomeDirectory()
                                   stringByAppendingPathComponent:@"Documents"];
    //最终合成输出路径
    NSString *outputFilePath =[documentsDirectory stringByAppendingPathComponent:@"output.mp4"];
    
    if([[NSFileManager defaultManager]fileExistsAtPath:outputFilePath])
        [[NSFileManager defaultManager]removeItemAtPath:outputFilePath error:nil];
    return outputFilePath;
}

@end