Two PCs in one room, each with a headset and a cable to the same router, on one subnet. One microphone sits between them, plugged into the host PC. The internet sits outside the room, struck through. — Two PCs in one room, on one router and one subnet. The internet sits outside the picture.

Two PCs in one room, one router, and a voice between them.

CO-OP Comms plays one microphone in two headsets: one on the host PC, one on a second PC in the same room. One PC hosts; the other joins from a list, or by typing its address.

It is not "Discord", and not an online service

Voice apps usually mean a service on the internet. CO-OP Comms has none.

No servers or relay
No accounts, sign-in or friends list
No lobbies or matchmaking
No recordings, telemetry or analytics in the app
No text chat: voice only

In short: no online service, made for the local network only. Both PCs must be in one room.

CO-OP CommsApp 5409710. Hosts and joins.

CO-OP Comms Client OnlyApp 5437680. Free. Joins only.

What you need