When you look at the "Console" window, do you see error messages like these " Failed to communicate with qtkitserver":
You can lauch the "Console" app from the "Utilities" folder in the Applications folder to check.
If yes, then you are having installed something incompatible that will prevent Aperture from importing and playing videos properly. The safest fix is to reinstall Mt. Lion. You do not need to wipe the system, just reinstall on top of the current system.
This problem usually occurs after cleaning cleaning caches with helper applications - then older and incompatible extensions will stop working. For a "quick and dirty"remedy see here: Re: Import video to Aperture library on Mountain Lion
But reinstalling would be amore solid fix.