How do you play background noise through your microphone?

Before for some reason, my microphone didnt pick up my voice. But it picked up my songs playing through my itunes. For example, i play this online multiplayer video game that makes it so that u can chat via mic. With itunes on, and while wearing a headset, my teammates and i can both hear the music and not my voice. How do i revert it so i can have this ability? I do not want them to hear my voice, but only the computer background noise. Thanks