Southwest Airlines announced a further ramp-up of its New Orleans and Chicago Midway schedules as part of 15 new flights announced yesterday. It will launch daily Las Vegas-Orlando service April 28, daily Midway-Orange County service May 25 and daily service from New Orleans to Ft. Lauderdale-Hollywood, Phoenix, Las Vegas, Baltimore/Washington International and Midway June 10. It also is adding one daily frequency on the following routes: Midway to Buffalo, Salt Lake City and Las Vegas from April 28; Las Vegas-Orange County from May 25; New Orleans-Houston Hobby, Seattle-Las Vegas and Long Island Islip-Las Vegas from June 10, and Midway-St. Louis from June 11.