Two sources (desktop PC + Mac) into one speaker setup?

[Scam]

Since I've started WFH I've been using a HDMI/peripheral switcher to switch between my personal PC and work Macbook which works rather well to be able to use my existing keyboard/mouse and monitor. I'm using my PC to play music etc. through my Creative speaker setup. It's some old 5.1 system but I only have 2 fronts coming out of the sub due to space. Fine by me.

Now, I realised that I could actually install Spotify on my work Macbook so it seems a bit silly to run my personal PC all day just for music. The Creative sub just has a 3.5mm input, so I'll be finding a 3.5mm cable to plug up my Macbook and should be set. But… I do have an old headphone splitter (1 male to 2 female). So can I plug both machines in at the same time?

tldr; Can I use my headphone splitter to plug in both my PC and Macbook into the Creative sub, so I don't need to scrabble under my desk should I want to listen to stuff out of my personal PC?

 

[Scam]

No such concern using a splitter the other way round though.

I didn't think I was in a position to test this because I only had a male-to-female extension (hence the thread), but I managed to find a male-to-male cable from my Chromecast Audio. I gave it a go but with both machines on the volume seemed to jump up and down especially when waking the Macbook.

Perhaps it was my jerry-rigged M>F>M>M>splitter extension though, and actually I'm not likely to have both machines on (that's the whole idea). I've ordered a M>M extension for the Macbook so I can test properly when that arrives. I guess I was just scared I was going to blow up the speakers or something?

 

[Scam]

Well this is highly annoying. I'm using a USB-C Digital AV Multiport Adapter to plug my Mac into power/hdmi switcher/kvm and it seems whenever this is plugged in I'm getting almighty signal noise through my speakers. Like, it's louder than the music I'm playing As soon as I unplug the adapter the audio is fine… I didn't have this last week when I was testing with my crazy extension!

This never happened before when I was testing with my ghetto cable as above, but with the same cables and/or my new straightforward 3.5mm extension to my speaker setup, it's happening every time so it's unusable. Any ideas? Why would the AV adapter give noise to the 3.5mm out on the other side of the Macbook?