Parece que o Steam, ou qualquer aplicativo que use o seu Mic pode causar esse problema, tente o que está escrito abaixo:
The reason why the music volume goes down is because Steam by default has activated the option "Automatically transmit my voice whenever I speak". So the solution is to:
1) Go to Steam > Settings > Voice tab and check the "Use a push-to-talk key to transmit voice".
2) Restart Steam and Audio app and you're done!
Extraído daqui: link