''Sit back, relax, and sharpen your fangs,'' the audience is advised in ''Forbidden Broadway,'' playing at the new Terrace Room at the Park Plaza Hotel. Fangs is right. This satirical parody of musical comedy has sharp teeth that nip at hits from ''The King and I'' to ''Cats.'' In front of a shimmering silver curtain, tuxedoed singers spoof Yul Brynner's bald head, Ann Miller's shatterproof hair, and Lauren Bacall's boyishly low voice.
The teeth, however, are small. Gerard Alessandrini has created 28 fast-paced numbers, none of them blazingly witty, most of them mildly clever, and some quite mean-spirited. And, as with most roasts, one not privy to the in-jokes will feel a bit left out.
The performers - spirited and fine-singing - do their best with the material. Karen Murphy has an elfish twinkle and a mobile face; Toni DiBuono a great Ethel Merman imitation and belt to boot. Jeff Bannon and Craig Wells have less definition but still carry it off.