In the System Preferences, take a look at Sound. You can change the default input or you can change the microphone sensitivity. I use a headset with Skype.. but the sound 'control panel' is a good place for you to start.
Well, I was actually on a call at that time, I guess I just can't adjust the 'sensitivity' while chatting? Or will it automatically go back to the default whenever I start using it again?