Depends on the parade, there's a couple different variations on the system they can use. In general though there's a background music loop that plays in each of the parade "blocks" and repeats over and over again for the entire parade. In some parades the loop will switch depending on the section of the parade. A block is a relatively small section of the parade route that plays the same audio through all the speakers in that block. For example, town square is a block, and the next block is main street itself. Blocks are easily identifiable when parades start and end, because you'll hear the opening and closing of the parade twice if you are near the borders. At the same time each float has its own music and dialog loop playing through its onboard system. The block "background" loop and the onboard loop are synced together.
In your example above the Peter Pan float would have a loop with Peter pan's music and dialog. It'd probably be a couple minutes long and keep repeating. At the same time the generic parade music loop would be playing throughout the entire block. The key thing would be that the loops are either the same length or multiples of each other in order to keep everything together.
The system can become more complex or less depending on how the parade was designed, whether there's show stops, one loop for the whole parade (think current day parade) or multiple loops (elp, or xmas parade) length, etc.
I don't necessarily have all the technical details down and am not sure how familiar you are with smpte but suffice it to say it all works off of synchronized timecode. Essentially there's a master clock and a clock for each subsystem. They're locked together to be the exact same time down to fractions of a second so that when one starts or changes the others stay in sync.
As far as to when the loops change over, its supposed to be automated based on tracking devices built into the parade route, but as far as i know the system seldom if ever works. Instead there are techs that walk the parade route and radio down to master control the position of the parade. Hope that helps.
