In the last years I've used this standard sound mixer (photo below) from one of the next online local electronics shops, powered by USB, to connect my personal computer, also my Chrome/Linuxbook and my microphone, so that I can hear them combined in my headphones. There are enough solutions out there, but at that time, I thought a small kind of professional, cheap (40 €), but cool looking mixer is something, that I need. And I was wrong (again).
In my daily life, I rarely use the microphone of my headset, so a few weeks ago I switched back to some of my old school headphones (cheap Behringer Studio Headphones HPS3000, you can get them between 10 to 20 € online) and dropped the headset. It's much more pleasant, because there is no annyoing, heavy and useless mic-controller on the cable anymore. That was the first step, but there were still some troubles with the volume of the USB connected mixer, depending on my different operation systems - because there was a huge difference, so this means I had to set the volume of my laptop to 100+% and my personal computer to 5% or 20%, to get a smooth, equally mix between them (for example to watch a movie, but also check a short news video on the other computer or get some acoustical software notifications). Also the switch between and control of Line/USB and other stuff doesn't felt likeable (for me).
So I switched back to a good, old school, analog-based IMG Stageline MMX-30 mixer with three stereo channels with Cinch connectors and adjustable output for every channel - including two headphone jacks and one, separated Cinch Master Output, protected by a black metal housing. Solid, big and heavy. Note: on the photo below I've just connected one device.
This should be enough for the next years and I really like it. The sound is very clear and heavy, without any disturbances and humming. And I also like the regular transformer power supply - a blast from the past. Now I can mix and control the sound of my computers without any unnecessary, fancy functions, sliders and useless USB~microphone support or to click around on the screen. I can only recommend this setup. Simplicity... \o/