Those files might be corrupt. If yes see this: A
http://www.quinnware.com/couple of people have suggested fixes to those so called corrupt files. There's apparently at least 2 utility apps out there that can fix many. I don't know about all. See these: 1.
video-fixer & 2.
MP3RepairTool
Else try a 3rd player that has in-built codecs & plays all (free): QCD:
http://www.quinnware.com/ I forgot the link to qcd lol.