I Don't Know if this has changed in the last fiew Years.
But between 1996 and 2000 (the last 2 times I was at WDW) the fireworks were shot of at the end of the "New Years Eve Show" as a Pop/Rock Version of "Auld Lang Sine" played.
The Show had 4 or 5 songs in it but I do not recall a real Theme Song.
The Fireworks at Pleasure Island are pritty short, I would say no longer than 5 minutes, the main attraction of the firework show is the fact that the fireworks are shot off from the buldings surounding the viewing area.
The PI fireworks have been shortened even more considerably since early 2002. The running joke at the Adventurers Club is that you can go outside and see the fireworks, which consist of a monkey with a sparkler.
Seriously, it's a VERY short (less than 3 minutes) show, with the fireworks now being blown from the roof of the Adventurers Club. They DO do longer sessions for holidays (July 4th, a day or two of Black History Month, Cinco de Mayo, etc.), but they're few and far between.