Subwoofer out on Dimension E521

I've got a Dimension E521.  I've upgraded the video card and added RAM.  All it's going to be used for is playing music and some web browsing.  I've got the basic dell speakers plugged into the speaker out jack in the back and they work.

I have an older stand alone IBM subwoofer that I'd like to add to the system. However, when I plug it into the center/subwoofer jack in the back, I get nothing. When I plug it into the headphone jack in the front I do get sound from it. 

How do I get this to work?