FaceTime calls

Hi all,
This is one thing I've noticed for quite a long time now and haven't really done anything about it sooner. When ever I am on a FaceTime call, every other audio including system audio, Skype calls, iTunes music and others are turned down to the point of being barely audible. I do not want this behaviour to occur. I tried looking under FaceTime preferences, and system preferences to adjust this but couldn't find anything. Can this behaviour be altered? I want to be able to listen to music while conversing on FaceTime.
Thanks