I don't know. You could try playing them in a different media player. VLC media player is the one that is usually recommended and is good to have as a backup. If it plays in that one then it was probably a codec problem. Other than that i don't know.

Edit: Could be you've been d/ling corrupt files. There are a lot of them. Try looking up the file with Bitzi first. How to use Bitzi Web Lookup .. & How to use Magnet Links & Direct Connect/Browse/Chat

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