The New Orleans Jazz and Heritage Festival crams all the musical strains of New Orleans and Louisiana-from brass bands to zydeco, and gospel choirs to the Neville Brothers-into two weekends of sweaty fun. Some fans park themselves in the Economy Hall Tent and soak up a solid day of traditional jazz. Others camp out before the big stages and score a good seat for the national headliners. Experienced festers drift around the fairgrounds and discover the unexpected-perhaps a nearly unknown local jazz prodigy, a gospel group made up of a dozen trombones, or a wild, witty, and profane man from Mississippi belting out obscene ditties.