Windows 7 Movie Maker-I need help slowing the speed of my video that has the voice speed like chipmunks(without unsyncing the movements & voices)

I created a 20 minute video with a webcam. When I played the video back in Windows 7 Movie Maker, it played back sounding like chipmunks. The speed was set at "1" before playing the video back and while I played the video back.  I tried to slow the video down by going to "Video Tools" and clicking on the upside down triangle next to the "Speed:" icon and changing the speed to .5. This slowed the speed down but the movements and the speed did not match. I also tried emailing the video to see if it would playback slower once I opened it. This didn't work either.

Also, before creating the 20 minute video, I created a test video that was about 2 minutes in length. When I played it back in Windows 7 Movie Maker, it played back in normal speed. I'm wondering if the length of the video has something to do with it. Do I need to slow the speed down by cutting the video in segments?

How do I fix the 20 minute video? I can't create a new video. I need to fix it.

Thanks for the help.