Aram sound bug?

Hello Riot! Since the 6.6 patch, when I am playing aram , the ambient sound , the bgm starts to flicker as though the game is fps lagging . But both the fps and the ping are fine and the gameplay is smooth, however the sound starts to begin flickering at the start of 3rd minute into the game! I was wondering if there is a problem with my system or if it is a bug! There' s no problem when I am playing the classic rift mode though!