You can go a couple ways with this one:
- Play the files in a different player. I use VLC - it’s your swiss army knife of media players on the Mac.
- Give QuickTime the codecs it needs to play the files. In this case, I’d use Perian. This will expand the file types that QuickTime can play. It’s a little old, but it does seem to still work in Snow Leopard.
Either solution works :)