音頻文件如果是錄音是錄音的時長即為音頻長度，有的時候需要獲取一個完成音頻文件的長度，判斷方法如下:

- (float)audioSoundDuration:(NSURL *)fileUrl {
    NSDictionary *options = @{AVURLAssetPreferPreciseDurationAndTimingKey: @YES};
    AVURLAsset *audioAsset = [AVURLAsset URLAssetWithURL:fileUrl options:options];
    CMTime audioDuration = audioAsset.duration;
    float audioDurationSeconds = CMTimeGetSeconds(audioDuration);
    return audioDurationSeconds;
}