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废话不多说,直接上代码

// 测试合成是否可用的(此方法可以无视)
+ (void)SyntheticRecordAndFinishBlock:(void(^)(BOOL success))block{
    NSString *main = [[NSBundle mainBundle] pathForResource:@"107785182" ofType:@"m4a"];
    NSString *change = [[NSBundle mainBundle] pathForResource:@"audio" ofType:@"m4a"];
    [self SyntheticRecordWithMainRecordFile:main AndChangeRecordFile:change AndChangeStarTime:10.0f AndFinishBlock:^(BOOL success) {
        if(success){
            //文件转移
            //删除文件
            block(success);
        }
    }];
}

/**
 * 第一段音频 0～30 秒  star 0  end 30
 * 第二段音频 10～15 秒 star 0  end 5
 * 合成: 第一段音频0~10 + 第二段音频 0～5 + 第一段音频 15~30
 *
 */
+ (BOOL)SyntheticRecordWithMainRecordFile:(NSString *)mainFilePathString AndChangeRecordFile:(NSString *)recordFilePathString AndChangeStarTime:(NSTimeInterval)starTime AndFinishBlock:(void(^)(BOOL success))block{
    AVMutableComposition *composition = [[AVMutableComposition alloc] init];
    AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    
    NSURL *mainFileURL = [NSURL fileURLWithPath:mainFilePathString];
    NSURL *changeFileURL = [NSURL fileURLWithPath:recordFilePathString];
    AVAsset *mainAVAsset = [AVURLAsset URLAssetWithURL:mainFileURL options:nil];
    AVAsset *changeAVAsset = [AVURLAsset URLAssetWithURL:changeFileURL options:nil];
    if (!([self AVAssetCanUseing:mainAVAsset] && [self AVAssetCanUseing:changeAVAsset])) return NO;
    //几个CMTime
    CMTime mainStar = kCMTimeZero;
    CMTime mainEnd = [mainAVAsset duration];
    CMTime changeStar = kCMTimeZero;
    CMTime changeEnd = [changeAVAsset duration];
    CMTime syntheticStar = CMTimeMake((int)(floor(starTime * 100)), 100);
    /**
     * CMTimeRange 用于剪切音频（三个或二个）针对一段完整的音频
     * CMTime 用于合成音频（三个或二个） 针对合成的音频
     －－－－＊＊＊－－－ 第一种  3段  1 2 1
     ＊＊＊－－－－－－－ 第二种  3段  1 2 1  第一段的 CMTimeRange ＝ 0
     －－－＊＊＊＊＊＊＊ 第三种  2段  1 2
     ***/
    //计算出三个 CMTimeRange
    CMTime firstStarTime = kCMTimeZero;
    CMTime twoStarTime = syntheticStar;
    CMTime threeStarTime = CMTimeAdd(syntheticStar,changeEnd);
    
    //拼接
    BOOL success = YES;
    
    CMTimeRange firstRangeInAsset = CMTimeRangeMake(mainStar, syntheticStar);
    if(starTime > 0){
        //非第二种情况才拼接第一段
        success = [self CompositionAudioTrackInsertWithCompositionAudioTrack:compositionAudioTrack AndStarTime:firstStarTime AndCMTimeRange:firstRangeInAsset AndAVAsset:mainAVAsset];
    }
    CMTimeRange twoRangeInAsset = CMTimeRangeMake(changeStar, changeEnd);//这一段就是第二个完整的音频
    success = [self CompositionAudioTrackInsertWithCompositionAudioTrack:compositionAudioTrack AndStarTime:twoStarTime AndCMTimeRange:twoRangeInAsset AndAVAsset:changeAVAsset];
    if(CMTimeGetSeconds(threeStarTime)<CMTimeGetSeconds(mainEnd)){
        //非第三种情况才拼接第三段
        CMTimeRange threeRangeInAsset = CMTimeRangeMake(threeStarTime, mainEnd);
        success = [self CompositionAudioTrackInsertWithCompositionAudioTrack:compositionAudioTrack AndStarTime:threeStarTime AndCMTimeRange:threeRangeInAsset AndAVAsset:mainAVAsset];
    }
    //生成音频文件
    success = [self SyntheticFileWith:composition AndFilePath:self.writePath AndFinishBlock:block];
    return success;
}

/**
 * 判断是否可用
 */
+ (BOOL)AVAssetCanUseing:(AVAsset *)avAsset{
    NSArray *tracks = [avAsset tracksWithMediaType:AVMediaTypeAudio];
    if ([tracks count] == 0) return NO;
    return YES;
}

/**
 * compositionAudioTrack 拼接
 *
 */
+ (BOOL)CompositionAudioTrackInsertWithCompositionAudioTrack:(AVMutableCompositionTrack *)compositionAudioTrack
                                                 AndStarTime:(CMTime)starTime
                                              AndCMTimeRange:(CMTimeRange)timeRangeInAsset
                                                  AndAVAsset:(AVAsset *)avAsset{
    AVAssetTrack *clipAudioTrack = [[avAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
    NSError *error = nil;
    BOOL ok = [compositionAudioTrack insertTimeRange:timeRangeInAsset ofTrack:clipAudioTrack atTime:starTime error:&error];
    if (!ok) {
        NSLog(@"Current Video Track Error: %@",error);
    }
    return ok;
}

/**
 * 合成 ???
 * 不会 覆盖 原文件 (存在原文件导致失败)
 */
+ (BOOL)SyntheticFileWith:(AVMutableComposition *)composition AndFilePath:(NSString *)mainFilePathString  AndFinishBlock:(void(^)(BOOL success))block{
    // create the export session
    // no need for a retain here, the session will be retained by the
    // completion handler since it is referenced there
    AVAssetExportSession *exportSession = [AVAssetExportSession
                                           exportSessionWithAsset:composition
                                           presetName:AVAssetExportPresetAppleM4A];
    if (nil == exportSession) return NO;
    //NSLog(@"Output file path - %@",soundOneNew);
    // configure export session  output with all our parameters
    exportSession.outputURL = [NSURL fileURLWithPath:mainFilePathString]; // output path
    exportSession.outputFileType = AVFileTypeAppleM4A; // output file type
    
    // perform the export
    [exportSession exportAsynchronouslyWithCompletionHandler:^{
        
        if (AVAssetExportSessionStatusCompleted == exportSession.status) {
            NSLog(@"AVAssetExportSessionStatusCompleted");
            block(YES);
        } else if (AVAssetExportSessionStatusFailed == exportSession.status) {
            // a failure may happen because of an event out of your control
            // for example, an interruption like a phone call comming in
            // make sure and handle this case appropriately
            NSLog(@"AVAssetExportSessionStatusFailed");
            block(NO);
        } else {
            NSLog(@"Export Session Status: %d", exportSession.status);
            block(NO);
        }
    }];
    return YES;
}

辅助方法

//文件路径
+ (NSString *)getAppDocumentDir
{
    return [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
                                                NSUserDomainMask,
                                                YES) objectAtIndex:0];
    
}