Clear audio when running a game, muffled audio when game is closed?

ok so when i have this game called ';diablo 2'; running, the audio sounds crystal clear. bg music, youtube videos, AIM sounds, etc they all sound really clear, but when i close diablo 2, everything becomes muffled. the audio sounds really distorted and stuff. i have all the latest drivers and this never happened before. it started last month. why is this and how do i fix it?