It’s hard to believe that 20 years have passed since John Belushi first impersonated a zit and killed a horse in Dean Wormer’s office. But to celebrate the auspicious anniversary of “Animal House,” Universal is rereleasing the film’s soundtrack (September 22), which features such college party classics as “Louie, Louie” and “Shout.” In addition to the music, this enhanced CD also contains a seven-minute video with behind-the-scenes footage of John Belushi fooling around on the set, interspersed with interviews with “Animal” alumni such as producer Ivan Reitman, director John Landis, co-writer Harold Ramis, and co-star Kevin Bacon. The interviews aren’t that revealing: “I wanted to hang out with them,” says Bacon, “but they wouldn’t hang out with me” is about as controversial as it gets. But these video bites are culled from a much longer retrospective, which will be available on the DVD release (October 13). Perhaps then we’ll find out all the true mysteries, like what really was under those togas?