Have you identified the Gadget Labs 8/24 sound cards as the source of the buzz, or are you still not sure? When hunting for problems like that, you have to basically test each piece of gear by itself. So that means either pulling one cable at a time, or pulling them all and then reconnecting one at a time.
You also need to eliminate your speakers and the wires to them, so you'll need to listen on headphones to make sure the hum isn't there.
Another common culprit is wall warts or lump in the line power supplies. You should unplug those one at a time to see if they are causing your hum issues. Are any cables running too close to unshielded monitors or any other source of interference?
Hunting down hums and buzzes is a painstaking process of elimination, I've done it a number of times. You should also pull one of the Gadget Labs 8/24 sound cards and test the other, then switch them, in order to identify whether they are the cause. If the sound is coming from both of them, then I would be suspect the source of the buzz could be elsewhere because it is less likely that two sound cards would have the same problem at the same time. Although I am not familiar with that sound card, and it may have some sort of problem. Were they working okay and then a buzz started, or are they brand new and you've had a buzz since installing them?
Also, if you added something to your setup recently and then a buzz started, that's the first piece of gear I would test as the source of the problem. Hope some of this helps.
