Noob Question

How many rooms (besides the living room) are wired for speakers? You said "all of them", but the total count is important. The photo of the wallplate doesn't tell us much - are those all the speaker wires? Separate out the ones for the living room. If you can unscrew one of the volume controls so we can get the brand/model, we'll know what you've got and what else you'll need.

In simplest terms, you just wire those speakers to an A/V receiver or amplifier. An AVR with "zone 2" support can give you independent control of those other rooms. Or an inexpensive AVR dedicated to the other rooms. In order to drive many speakers (rooms) from a single amp, you have to use "impedance matching" so the amplifier isn't driven too hard by all those speakers...

How much flexibility do you want in those other rooms? If you want to have different sources playing at the same time, we're into the "whole house audio" category.

All depends on what you want to accomplish and of course, the budget...

Jeff

Have you pulled the plate off where the 2 wires come out? Have a look and see if there are any more wires that maybe aren't pulled out. You could also have a look in the utility room by the electrical panel. The wires should be accessible from somewhere, is the builder available for that info?