I’m just being pedantic here, but an intercom system expands anywhere there’s networking, requires fewer wires, and lower voltage (will even work without power, not very well, but it will)

Did you run the line under ground or through conduit? Disconnect it from both sides, attach string/fishing line to one side, pull cable out of conduit on the side without the string attached. Detach string from either meet cable, reattach string with Ethernet cable and the additional cable (speaker cable in this case), pull string from the original side until both cables make their appearance.

Outside of walkie talkies (have you considered walkie talkie? Maybe put an antenna outside of the shop?) this is the simplest way to do it (imo, but I don’t play with raspberry pi’s)

It’s literally speaker wire, or you could clip the ends of your Ethernet and use those (but you probably want your Ethernet out there still XD. One side will be to be run to power, but it’ll power the rest of the stations through the wire.

I feel like this is definitely what you’re looking for, no computers required.

From Amazon, I found “IS543 5/4/3 WIRE INTERCOM STAT-PLAST”

It’s similar to what we had growing up, 1 in the shed, 1 in the kitchen 1 in the game room, push the button of the location you wanted to talk to, only 1 direction of communication at a time, over.

I feel like you’re making it way more complicated than it needs to be, why would you involve a switch or processor when you’re just carrying sound waves? You don’t even need a modulator, it just needs to connect a microphone to a speaker like cans on a string. Is it the highest quality? No, will it record your conversation for posterity? Also no. Will it let you tell at someone to get their asses in for dinner, absolutely

You’ve already run wires, why not a microphone and speaker system?

You don’t have to be the most private, just don’t draw the most attention…

Big data is an unfortunate cost of society.