I run two (sometimes three) PCs through a self-made resistive mixer with four inputs. Works fine enough most of the time, but some cards with crap sound output seems to worsen with this and pick up noise. In retrospect, it might have been better to have used 1K resistors instead of the 10k I used, as lower resistances are supposed to be a bit more resilient to that (from what I've read), but I was a bit overprotective of loading the sound outputs too much when I do have four inputs connected, as the more inputs you add, the lower the impedance is.
I'm thinking about either making or buying an active mixer. A switch isn't a good option for me as I do want to hear all PCs at the same time (listening to music in my main PC while setting up the retro ones, etc.)

An op-amp based unity gain mixer would be an inexpensive and relatively easy upgrade. It would definitely give you more ideal behavior in terms of impedances.

I use one of these active mixers, seems to work OK but only 3 stereo channels. It comes up cheap on UK eBay from time to time, I think I paid less than £20.
http://www.pulse-audio.co.uk/5051259014879.shtml

Does the Art Splitmix 4 have 4 stereo channels, or 4 mono channels?
Would love a Roland MX-5 😀

I gave up having my MT-32/CM-32L/SC-55 all connected at the same time, too many wires and power bricks going on.
Now I just have 1 PSU, MIDI cable and audio cable and swap them around as needed.

The Art Splitmix is 4 stereo channels, however it uses 6.35mm jacks so it's a bit annoying as the adapters take more footprint at the back.

I hear you about using multiple modules - at one point I tried having CM-32L / CM-32P / SC-55 / MU80 all hooked up to an MPU-IPC-T. I got quickly fed up with the mess of cables, and too many external boxes and power bricks lying around. Now I just use one module and the others are stored.

I just use a 4 port KVM with Audio, USB, PS2, and VGA.