参考连接:http://msching.github.io/blog/2014/07/09/audio-in-ios-3/

计算总时长

计算时长Duration
获取时长的最佳方法是从ID3信息中去读取，那样是最准确的。如果ID3信息中没有存，那就依赖于文件头中的信息去计算了。

计算duration的公式如下：
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double duration = (audioDataByteCount * 8) / bitRate
音频数据的字节总量audioDataByteCount可以通过kAudioFileStreamProperty_AudioDataByteCount获取，码率bitRate可以通过kAudioFileStreamProperty_BitRate获取也可以通过Parse一部分数据后计算平均码率来得到。
对于CBR数据来说用这样的计算方法的duration会比较准确，对于VBR数据就不好说了。所以对于VBR数据来说，最好是能够从ID3信息中获取到duration，获取不到再想办法通过计算平均码率的途径来计算duration。

获取ID3信息:

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- (void)getDurationWithCompletion:(void(^)(NSTimeInterval duration))comp {
    NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
        AVURLAsset *audioAsset = [[[AVURLAsset alloc] initWithURL:_url options:options] autorelease];
    NSArray *keys = [[NSArray alloc] initWithObjects:@"duration", nil];
    [audioAsset loadValuesAsynchronouslyForKeys:keys completionHandler:^{
        NSError *error = nil;
        switch ([audioAsset statusOfValueForKey:@"duration" error:&error]) {
                
            case AVKeyValueStatusLoaded:{
                CMTime audioDuration = audioAsset.duration;
                
                NSTimeInterval duration = CMTimeGetSeconds(audioDuration);
                 if (comp) {
                     dispatch_async(dispatch_get_main_queue(),^{
                         comp(duration);
                     });
                }
            }
                // duration is now known, so we can fetch it without blocking
//                CMTime duration = [asset duration];
                // dispatch a block to the main thread that updates the display of asset duration in my user interface,
                // or do something else interesting with it
            default: ;
                // something went wrong; depending on what it was, we may want to dispatch a
                // block to the main thread to report th         e error
        }
    }];
}