Might be that the files you've downloaded are fake. The network is full of them, but you can learn to avoid them and find good ones. Look up the files with
Bitzi before downloading to help sort out the bad ones.
See:
Recognising Fake files
And for tips on finding good files, see:
How to find music..
Also did you try VLC Media Player?? And ALWAYS use BITZI to check for good Files. Hope this helps.