I have a powerbook and I can't seem to play a lot of the mp4s. Sometimes they open with a white screen and other times I get an error: "invalid sample description was found in the movie"
I'm using quicktime 7.2 I would think that mp4 is the one thing quicktime should always open.

I don’t know if this will help but you should try reinstalling QuickTime.

I have a powerbook and I can't seem to play a lot of the mp4s. Sometimes they open with a white screen and other times I get an error: "invalid sample description was found in the movie"
I'm using quicktime 7.2 I would think that mp4 is the one thing quicktime should always open.

This will sound simplistic but try reloading the page. I don't know if I read the explanation correctly but the computer may require a reload, first comes the sound, then comes the picture.

May be it's a codec problem?
download latest Quicktime Alternative codec
http://www.free-codecs.com/download/QuickTime_Alternative_Lite.htm

Try changing the extension to 3GP. Did you capture your video with a cell phone?