I have had this problem for a while now. I am using a pair of Razer Kraken Pro V2's but this problem has persisted with other headsets. My microphone picks up any output volume even when muted, resulting in it being really annoying for people that I speak to over voice communications. I'm using the headphone/mic splitter provided which is plugged into my motherboard IO. I just use the built in sound card of my ASUS Z270E motherboard. I have even tried to use a USB device that has the splitter ports and the problem still persists. In terms of settings, Stereo Mix has been disabled and I have made sure that "listen to this device is off" in the microphone settings. I'm really starting to get overly annoyed by this.