Losing sound in-game after some time idle

I have a LG OLED connected to a Marantz Cinema 50. My gaming PC is connected to the Marantz and all works like a charm. But when i leave the game for about 5 minutes and do nothing (only the background music is playing), i lose all sound from the game. I have to restart the game to get music back. When the game is running in background without sound and i go to e.g. youtube, i hear the youtube sound, but the game sound is still dead. Anyone an idea what is that?