Maybe try
QuickTime 6? If you have it already, fire-up the "quicktime updater" to make sure that you have the latest CODECs. I saw the audio only problem when I tried to play the medium resolution version on my ancient 266 MHz laptop, but found that it works on all of my faster machines. I'm theorizing that the video bit-rate is too high for some systems and QuickTime, knowing that it can't display the video portion adequately, falls back to an audio-only mode.
-Kevin