During a call, I can hear the sound from my friend's PC, but not all applications.

There's a guy in our circle of friends, someone we're often on calls with, who has a real problem with transmitted sound.

We can hear the sound from certain applications very clearly when he's on a call.

It might sound crazy, but here's what we know:

• This friend has a headset with a microphone that he can unplug with a button to mute himself.
• We've played a lot of games, yet we can hear the sounds from YouTube (browser), some gacha games, and Minecraft, but not Repo, Apex Legends, The Finals, Ryujinx, or any other game, even full-screen games.
• It only happens on Discord (when we're in voice chat during a game, it doesn't happen, even though he sometimes watches videos on Discord). (side)
• He has noise cancellation (Krisp) enabled.
• He can hear the sounds in his headphones, yet it's like the microphone is recording what he hears; sometimes it's perfectly clear. At first, I thought it was related to full-screen games, but there are some games that his microphone doesn't record.

We checked his input/output devices; it's just his headphones that are the problem. Any ideas?