For a generation of rock and roll fans it is astonishing news: the music loved by their hopelessly square parents has suddenly gotten hip again. Long ago vanquished by electric guitar-toting rockers, the big band swing music that once hogged the family hi-fi is back, providing the soundtrack for films, a nationwide swing dance craze and even khaki-clad kids doing the Lindy Hop in a Gap television ad. Leading the charge, along with Royal Crown Revue and Big Bad Voodoo Daddy, is this seven-member neo-swing orchestra. The Zippers bring swing out of mothballs with banjo, piano, horns, reeds, upright bass, Louis Jordan-style arrangements and a singer, Katharine Whalen, who does a fair impression of Billie Holiday. But why buy this when recordings by Benny Goodman, Count Basie and all the great swing bands of the '30s and '40s exist on CD? Maybe because the Zippers and their fellow retrogrades are making the music pop again.