A spin through the musical genres presently represented on the nostalgic jukebox that is Broadway will land you on shows built around classic rockabilly and early rock (''Million Dollar Quartet''), hair metal (''Rock of Ages''), the disco-pop of ABBA (''Mamma Mia!'') and the modern punk of Green Day (''American Idiot''). Is there room for one more show this season whose score you'll already know before you walk through the door, and if so, are there any musical subcategories that aren't represented yet? The producers of ''Baby It's You!'' -- a new musical planned for a Broadway opening this spring -- are hoping for affirmative answers to both questions. Their show, conceived by Floyd Mutrux, written by Mr. Mutrux and Colin Escott (both of ''Million Dollar Quartet''), and directed by Mr. Mutrux and Sheldon Epps, will focus on the pop and doo-wop singles of Scepter Records, the vintage-1960s label founded by Florence Greenberg, who helped discover recording artists like the Shirelles, the Kingsmen, the Isley Brothers and Dionne Warwick. [ABSTRACT FROM PUBLISHER]