How the mighty have fallen. Ratings for Megyn Kelly’s newsmagazine show, Sunday Night with Megyn Kelly, have steadily declined since its premiere on June 4 — to the point where the July 16 episode drew only 3.1 million viewers, half the audience that watched her debut. And with her 9 a.m. talk show set to launch Sept. 25, sources say NBC execs are quaking in their boots. After hiring the newscaster away from Fox for a reported $17.5 million a year, “they were expecting fireworks,” says an insider. But even her controversial interview with Sandy Hook denier Alex Jones lost out to a rerun of 60 Minutes. Now, says the insider, NBC is scrambling to figure out what needs fixing. “They’re hoping they haven’t made a huge mistake,” says the source.