When you are using DMX and then tell your lights to go into SOUND ACTIVE mode(again, you're controlling them in MyDMX), each light responds to it's OWN microphone, and hence responds independently.
Your best bet is to run them as master/slaves so they all listen to the first fixture, but then you can't use them as DMX fixtures.
Hobson's idea involves using a DMX controller, such as a DMX Operator, or in his case a Magic 260. It should work fine.
My idea requires significantly more cost and so I won't discuss it. It does still involve some of the stuff Hobson describes, just ultimately done via PC-based software instead of a hardware controller.
Jingles is right in the fact that it could all happen by random.
In one scene in a big show I do, I'm actually relying on these fixtures behaving independently to make the scene work. I also set the sensitivities different to ensure randomness. I have 8 64 LED Pros, and I get the results I wanted, which is a lack of consistency and lots of bright flashing colors. Flawless every single time! It's high-energy and the lights help bring it out!
The only way you're going to do what you want to do can't be done the way you want to do it. You have to take control of it one way or another to make it happen. The key is to make it appear random. Good luck there. But if you can take the control, then the rest is just trial and error until you like the results