Where can. I hire a basic sound system + 1 big speaker for a party in pattaya?

Can't help directly as we're not in Patters, but it's worth talking to your local Wat.

We've borrowed all sorts of stuff, tables, cutlery and crockery etc. all for a very nominal fee. They even deliver and remove. Never borrowed their sound system (which is excellent by the way) as we can hear it just fine from it's normal location

Rent a Thai girl first (preferably a local). Most of us use our Thai wives/girlfriends for this day-to-day type of stuff

There are a couple of places along Thepprasit that do this, also several on Pattaya 3rd Road. Just about any place with large speakers outside will rent out this equipment, or will know someone who does.