Why can I hear my voice in the headphones through the interlocutor?

Question

Headphones

The situation is this. My friend is sitting in Kingston hyperx Drone headphones, they are 4-pin with a splitter.
And when we sit in a crowd in discord and talk, then each of us sometimes hears his voice through that very comrade.
Further, if he disconnects the microphone built into the headset and connects any other microphone to the sound card, then this problem disappears. Any thoughts on this? There is another 4-pin headset and everything is the same

Answer 1

Well, apparently, the sound from the headphones gets into the microphone.