Sonic announces the appointment of Christina Bell Vaughan as president of Sonic Restaurants, Inc.   OKLAHOMA CITY, 2017-Jan-25 — /EPR Retail News/ — Sonic Corp. (NASDAQ: SONC), the nation's largest chain of drive-in restaurants, announced the appointment of Christina Bell Vaughan as president of Sonic Restaurants, Inc. (SRI), the company's wholly owned subsidiary of drive-in restaurants. Vaughan is a…