https://segmentfault.com/a/1190000002401961  音量调节https://www.google.com/?gws_rd=ssl#newwindow=1&q=ios+%E9%9F%B3%E9%87%8F%E6%8E%A7%E5%88%B6https://www.oschina.net/ios/codingList/399/ios-video?sort=view http://www.fantageek.com/blog/2014/09/07/avfounda