Doesn't it say anything when you try to open said files?
Like, giving you a new window saying that QT lacks some codecs to play the file and that those codecs might be available on the QT site?
Anyway, even if it doesn't, I still suggest downloading VLC and/or Mplayer (both free. You can find them
here and
here respecitvely).
Those two can play virtually any video/audio format and have features that Quicktime doesn't, unless you go Pro (like Fullscreen).