I was looking through google's images... did that adapter come with your z-640's?
I don't have this system so I wasn't aware it came with that adapter.
I think your problem is that you're giving a signal to your fronts and rears but not feeding it a center signal. The sub works because the speakers have an internal crossover and are feeding the sub the low parts of the input it is getting... but the other half of the yellow/brown jack (the center/sub one) isn't getting a signal.
So, you need to get the center a signal. I guess you'd want a mix of the stereo front signal (or stereo back since that adapter is giving you the same signal to the back speakers it appears).
The easiest way to do this is if your motherboard was able to take a line input signal and map it to 5.1 like I mentioned earlier, but I have no idea if it's able to do that.
Another option might be to use the digital out of the x-box (I don't have one, but they have a digital out, right?) and then buy a soundcard that can take a digital in... but that would probably be pretty expensive.
That leaves you with more adapter stuff. I think you'd need to split the signal coming out of the adapter with a y-adapter and then take that stereo signal and somehow combine it and feed it to the center/sub input on the speakers. I've never done this before. Would a simple stereo to mono adapter do this? If so, then you'd just need to add a 1 male to 2 female y-splitter into one of the outputs on the adapter box (the green one let's say). Then you would plug the stereo to mono adapter into one of the outputs. The other half of that y-splitter you'd hook-up the 2 male to 1 female y-splitter you already have hooked up for the fronts. For the end that you just changed to mono, you'd need to get another 2 male to 1 female adapter and plug one of the stereo inputs into the mono adapter and the other into your center/sub output of your motherboard.
This working is dependent on a few things that I'm not sure about.
1. The stereo to mono adapter combines the signals into a mix of both
2. A stereo plug will fit into a mono jack and split the signal back into two duplicate signals
I've never used mono stuff, so I'm not quite sure how it works, but I'd imagine that's how they work. I wouldn't try this until someone else tells you this would work.
If you don't understand what I wrote, I can draw you a quick little paint diagram if you want.