I had a similar experience. Some friends and I were doing the Deathsinger checkpoint and needed another Guardian. This random joined us, seemingly without a mic. We were talking strategy in the beginning. Since we thought he wasn't able to speak, one of my friends asked him to wave if he understood. He waved. Everything went according to plan. We were giving him grief in the end about no mic, just friendly stuff. Suddenly he started taking (squeaking) and said "I leave my mic off because no one wants to hear me talk". I felt kinda bad for the kid. He did a good job. It was nice to see someone so young have consideration for other people's feelings. He left before we could say "thanks" and "good job".