The one I mentioned is from steelseries, but there should be sound mixers available which work via software connection or maybe you can try to get it made with a raspi or another sbc, either yourself or by asking a friend who would be interested in making that for you.
I used to have a pair of headphones that did this a long time ago but I’ve never seen anything like it since. Idek what headphones they were…
Idk why nobody has made something similar that just works with any headphones with a 9mm jack :(
The one I mentioned is from steelseries, but there should be sound mixers available which work via software connection or maybe you can try to get it made with a raspi or another sbc, either yourself or by asking a friend who would be interested in making that for you.