I think it's because of the way it's compressed. It's like ten, 10MB files on top of eachother. On Mac (I'm assuming that you're using Mac) it only plays the first 10 seconds.

There is a program called Rosetta that works to fix the file so you can see the whole thing. I'm not sure if it's shareware or freeware. But, it's worth it if there was a cost.

I know I posted about it months ago. Maybe try searching on these forums for "Rosetta" or "only plays 10 seconds". It's been discussed before.

Good luck.