Hey guys,
Thanks for your help. The input on ST3 was indeed turned up, as we typically control the volume output through the iPhone. Also, ST1 is on a ducker with the vocal mic channel so that when the pastor speaks it will limit the background music, but I guess the CB radio was just blasted the input. It came through quite loud. The cable from the airport router to ST1 (sorry, my mistake) was just a cheap 3.5mm to mono TS splitter from amazon. It is probably not shileded, but it is new. This sounds like it is the most likely explanation. You guys are great!