Its a known issue that the team are still investigating, think the problem is they can't reproduce the bug, neither can I. After talking to one of the community staff one day I thought I would have a crack at it but no matter what I did I could not replicate the bug, seems to be totally random!
I just got this issue too but it was after my laptop drive had to be replaced. I installed iTunes and Spotify and it found my music but certain tracks (all mp4s) would not play. I am trying the QT install now since I have not put it on the new Windows build and will see if that clears it all up.
EDIT: yeah it all works now after installing QT for Windows. Thanks!