Samsung Note 4 apps will turn silent after a notification alert sounds

A

AC Question

I just started using a brand new Samsung Note 4 this year. I noticed that whenever a notification alert comes in (whatsapp, sms, email, plugging in usb charger, etc) while I am playing a game app (any game), the game app will turn silent immediately and the only way to enable the sound is to restart the game app.

The same problem happened before and after the update to Marshmallow.

I came across the same question in this forum but no answers were provided. Please help to resolve this problem. Thanks!