There are two standards for this: WEBN in Cincinnati and KGB in San Diego. Both stations have been doing fireworks shows with synchronized on-air soundtracks for over 30 years, and here's the important part: neither one is a July 4th show. Both are station-based events later in the summer.
Here's everything you need to know about KGB's SkyShow:
http://www.101kgb.com/cc-common/mainheadlines2.html?feed=107937&article=2355629
WEBN also had a web page with a history of their fireworks, but it doesn't seem to be there now. Their tech story, though, is pretty much the same as the KGB story.
Both stations do all the music. In the early years, the technology pretty much required that the fireworks company plan the sequence of events, and the station build a soundtrack in response to that. But in the past decade that has managed to be reversed. Now a station can assemble a soundtrack and the fireworks company can program their computers to fire off some really precise stuff in response to power chords, etc.
At least I know that's how WEBN does it now. A guy named Joel Moss has done their soundtrack since the mid-80's. Their show happens every year on the Sunday night before Labor Day (the show originally happened for the station's 10th birthday, which is in late August). I've seen most of their shows, and saw KGB's about 8 years ago in Qualcomm Stadium. It happens after a Padres game and you can buy tickets to be inside the stadium, or pay to park in the large parking lot, or go to a party on one of the many hills surrounding the stadium.
WEBN's show happens on the Ohio River from several barges, and zillions gather on the banks for free, although there's lots of overpriced parking and concessions. Like San Diego, many hills provide a good vantage point for miles around.