从视频中提取音频

AVAsset *asset = [AVAsset assetWithURL:[NSURL URLWithString:_videoUrl]];

    //    float startTime = 0;

    //    float endTime = CMTimeGetSeconds(asset.duration);

     NSDateFormatter *formatter = [[NSDateFormatter alloc] init];

    formatter.dateFormat = @"yyyyMMddHHmmss";

    NSString *str = [formatter stringFromDate:[NSDate date]];

    NSString *audioPath = [NSTemporaryDirectory() stringByAppendingFormat:@"%@.mp3",str];

    CMTime start = CMTimeMakeWithSeconds(0, asset.duration.timescale);

    CMTime duration = asset.duration;

    CMTimeRange range = CMTimeRangeMake(start, duration);

    AVAssetExportSession *exportSession=[AVAssetExportSession exportSessionWithAsset:asset presetName:AVAssetExportPresetAppleM4A];

    exportSession.outputURL=[NSURL fileURLWithPath:audioPath];

    exportSession.outputFileType=AVFileTypeAppleM4A;

    exportSession.timeRange= range;

    [exportSession exportAsynchronouslyWithCompletionHandler:^{

        if (exportSession.status==AVAssetExportSessionStatusFailed) {

            NSLog(@"failed");

        }

        else {

            NSLog(@"AudioLocation : %@",audioPath);

        }

    }];