[plain fan]

I'd take the time to determine exactly where the wires go from that switch. Based on your description I would guess that it is a whole house audio setup (of that era) and that box controls where the sound is going throughout the house. I'd hook a receiver up to that box, turn the volume down low, and begin testing the switches.

Then I'd evaluate what you want to achieve with music in your house. Many people are ditching whole house audio like that because they prefer the ease and convenience of bluetooth wireless speakers (Sonos, Apple, Google, Klipsch, KEF, etc). You will possibly decide that you want to rip out all of the speakers and just repair the drywall.

In the main room you may want to keep the speakers "as is" and add a receiver for your own material.