I just found this post and I tried it, and it worked for me. I've been searching for 2 days for a solution to this problem, so out of kindness I am reposting this answer to questions that still are unsolved.
"I FIGURED IT OUT!!! All you have to do is create an event. Go to File and click "New Event". Label it and drag whatever clips you want to preview. They all seem to work as long as they are in separate events."
So create a new event (not project, event). Highlight whatever clips you want to play with and drag them into that new event. Then go back to your project and look for clips from that event, not all iPhoto videos, just that event. You will hear them, and can now continue on.
i'm having trouble understanding. i created a new Event called Testing. "drag whatever clips you want to preview" ...? i can't drag anything from the Project window into an Event. if i try to cut and paste clips from my original Event that the Project is made from, let's call it Tango, nothing happens.
i can't move all the original clips from Tango to Testing, because i have many other movies that use the original Event, Tango. i can't just chop the stuff out of there.
i may be misinterpreting everything...