I'm not sure if this is the correct board but I had problems playing mkv files. The audio and the video doesn't seem to synch. It's faster than the other.
Either:
If 1. Then redownload. (Obviously)
If 2. Then Get VLC, CCCP, install, restart, try again.
If 3. Then FFFFFFFFUUUUU-
Alternatively(Else If), you could convert it with a freeware program such as MediaCoder, and mess with the sync from there. AnyVideoConverter also fixes sync problems, assuming you know the offset between the audio and video streams.
Try an other media player. I´ve had Problems with VLC-Player. K-Lite Codec Pack with media player classic works fine.