I stole this from the original poster (John Greythorne) on the SonikMatter website.
Go into the "/System/Library/CoreServices" folder and control-click on "Setup Assistant". Click on "Show Package Contents".
In the package, go into the "Contents" folder and then the "Resources" folder. Control-click on "TransitionSection.bundle" and click on "Show Package Contents".
Again, go into the "Contents" folder and then the "Resources" folder.
You'll find the "intro-sound.mp3" and "intro.mov" files there.