Forum:Mute sound when window lose focus?

Is there anyway to mute Borderlands when I Alt-tab out of it? I personally don't enjoy Borderlands' background music (the one that plays when player is travelling/idle), and I'm find it hard to concentrate with the music playing in the background. There should be a more convenientt way of muting the sound other than turning my speakers on and off. Sins of a Solar Empire actually gives you a option to mute sound when window loses focus (great BGM by the way) and countless other games automatically does that.