Multiple areas need sound systems.
The Big top needs one for any shows; renegade, Open stage, BYJotY, etc. This does not need to be a huge system, a 1kW system with minijack inputs for MP3 players, ipods, etc, and a twin CD player will suffice. This also requires 2 microphones.
The Bar tent needs a sound system for ambient music, microphone for any pub quizzes, etc., be careful not to overdo this system, the bar does not need 3kW or more, it will run happily on under 1kW. In fact even cranking up 1kw will upset a lot of the drinkers/talkers/game players.
If you have bands playing, they may have specific requirements for the sound system - double check this before you book them. Ideally they would bring all their own equipment, but they may well not. Whoever supplies the equipment, you need to make sure there is enough power to plug it all in. The band will also need someone to babysit them during setup.
The juggling hall needs some background music.
Andrew Farr brought his sound system for 2013 and provided the choons. The choons were good. He would be willing to do the same again.
A mobile (battery-powered system) is useful for the games, workshops, etc..
It's not trivial to move a sound system, so you need to work out how much sound equipment you have, decide where it will go and then program your events around this. You don't want to have three spaces needing audio if you only have access to two systems.
Many successful BJCs have said thank you to Mini Mansell by donating some sound equipment to him. This means he has two sound systems and a battery powered amp, and can loan these to a BJC for free.
You can also hire gear in.