Behind the Scenes:
Go to the track listing for the videos. Go to the credits icon on the right hand side of the screen and hit enter. While the credits are rolling, press enter again and it will go into a short video of the voice actresses for the Knight Sabers singing a song together.
Reported by: Kevin

Bonus Feature:
From the track listing for the "music videos" included on the third disk of the M2K BGC DVD collection, press the right arrow. If it works on your player as it did on mine, the cell art gallery will come up in a slideshow with the song "Victory" in the background.
Reported by: Kevin