I've got a set of Cyber Acoustics 3602 speakers, and its a 2.1 system with a control thingy. The way it is setup is the source (computer, ipod, etc. goes via 3.5mm to the controller, which then goes to some unknown plug into the subwoofer, which then outputs 3.5mm to the speakers. I'm redoing my sound setup for next semester since I have my own room now, adding some other speakers I've got at home via the subwoofer output. This just brought about my curiosity on this subwoofer port. All I can tell is it resembles an s-video or ps2 plug... it has 9 leads and the case is red. Second page of this pdf you can see all that I know about the plug. http://www.cyberacoustics.com/app.cfm?fa=resource.dl&file=ca 3602_manu09_4th.pdf
The link downloads a pdf. The plug/port you're looking at is a DIN connector. Or actually, a Mini-DIN, like S-Video and PS/2.