Reply to Numark DJ IO problems

Good afternoon, great site, hopefully you can help me with what will be a simple question.

I have purchased a Numark DJ IO and I want to use this in conjunction with a pair of headphones on my desktop.

I start playing a song on my pc and the sound is outputted through my pc sound card and speakers. When I plug in the DJ IO it takes over from the pc soundcard and the speakers are muted. I know that the sound is being transmitted to the DJ IO as it shows up in the sound options in control panel as below: -

http://img.photobucket.com/albums/v736/ ... 0e0c40.jpg

The problem I have is that no sound comes out of the headphones. Is there something else I need to do? Or do I have a faulty unit?

Thanks for taking the time to read this.