Question.1  :I can't play a the download video file. What's wrong?
Question.2  :The picture is O.K., but can not hear any sound.Why?
Question.3  :When I tried to play it, a message came up saying" Content Blocked. Download **** Converter a free program to watch movie or call some weird phone number to obtain a password ?what i can do?
Question.4 : video and audio is out of sync!!!

With the k-lite codec pack installed, Media Player Classic can open just about any video file you can think of with the exception of realplayer formats.
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Here's a fair list of extension types it can open:
mkv
ogm
avi
xvid
x264 \ basically the same
h264 /
mp4
3gp

and a bunch of others all of us have never heard of or encountered. If your computer is good enough, I'd recommend watching h264/x264 format videos since they're AMAZINGLY high quality for their file size (they just take a good processor though).

And VLC Player is NOT recommeneded.

Question.5  :Very low sound when play the film? ?

Please just adjust the AC3Filter. And you can visit http://www.videohelp.com for some more details on how to do that.

Question.6  :How to use subtitles ? ?
You should have installed the k-lite. Your subtitle file(s) should have the same filename as your video file (for example 1: MovieName.avi + MovieName.idx + MovieName.sub)(for example 2: MovieName.avi+MovieName.srt) and should be in the same folder as your video file. Then the subtitles will automatically be displayed in most players.