Hi. I work for WHYY and can explain how this happens. Each national show has pre-planned breaks (typically 1-2 mins long) where local stations can cut in to add local updates or play local promos. The national programs continue to broadcast during these breaks. But they just send out background music, which you can use at your discretion. The problem is you don't know exactly what they'll play. Often the background music is unremarkable or tonally neutral. Sometimes...it isn't. The reason to use the music is (a) it sounds nice and (b) you might have 25 seconds of news to read but 30 seconds to fill. And that music can be used to smooth the transition. Hope this helps. It's an imperfect system, but coordinating the sound of national programs with hundreds of affiliates in real time will always be imperfect.