Double checking the files by trying to play them with the free VLC player which comes with its own built-in codecs could determine whether it is indeed a file or a player problem.
http://www.videolan.org/vlc/
As for codecs, I'd reckon using either the K-lite codecs pack or the Tsunami.
http://www.k-litecodecpack.com/ http://www.freetodownload.com/downlo...ami-Codec.html 