Friends mic suddenly goes quiet

Okay so I was just playing Junkensteins Revenge with a friend. We had played a few matches where we were able to talk via voice-chat just fine. Then suddenly his mic went really quiet. I was still able to hear that he was saying something, but I was unable to understand anything he said.
All options are as they were before, and I didn’t change any options either.
Windows, Discord etc. all pick up the sound as normal, only overwatch seems to have problems with it.

What can he do to fix this problem??

(sorry for my bad English)