Max has an iMac running Mountain Lion and iMovie doesn't allow him to play movies and it's running really slow. Leo recommends reinstalling iMovie. He should uninstall it, then go to the App Store and download and install it again. That often cures a lot of problems.
Leo would first edit the video in iMovie. In there, he can remove the sound track and remix it with music. When he exports the video, he should check the export settings to make sure it's set to export at the highest quality. Josh, in the chatroom says it's easy to "squish the wind noise" by using the equalizer and cutting it by 6DB. That'll lower the wind noise without eliminating the rest of the sound track.
Green screens are commonly used by meteorologists when they do their forecasts. They are stand in front of a green screen which is replaced by the camera with a weather map. This is called "matte-ing" or "keying". Alec wants to use this for his stop motion LEGO videos, and Leo gives him a couple of options: