So purpose of the fire was to send a signal to anyone on a boat or ship to come rescue the boys. However, later on in the story, someone sets the jungle on fire to kill Ralph. So, the fire was originally intended for good but then was used to do evil.
As for the drum roll, I think it represents chaos and future troubles they will encounter because they have "fallen apart as a society." There is a lot of discord among the boys.
Hopefully this helped and good luck.
It is Expository I guess.
