What are you even doing where you have multiple PCs where you use the audio in one spot?
The only thing I have done that with is my work laptop and my home PC. I used to have them connected to the same receiver and swap cables for the microphone, but I just use a separate headset for my work laptop and it works better that way. I never listen to any audio on my work computer unless it's a meeting and just have all the notification sounds play through the laptop speakers or a monitor.
My workstation where I spent most of the time is Linux. I have an additional PC that either boots into Windows for gaming, or other OSes for development. Then there is a laptop that sometimes wants to play audio.
Way back when I had two computers I wanted to hear, I had an old tandy speaker thing with two stereo inputs and a mix knob; but it died at some point. Now, I just have little speakers at the computers. TVs get receivers and decent speakers, the htpc went through the receiver until the htpc became too much pain.