My first thought would be to try moving the speaker wires around a bit. If the wires are lying close to a switching ciruit, it's possible that it's picking up noise.

Did you connect the original ground wire or the new one that comes with the ColorDMD kit? (Or neither?)

Hum is generally produced by ground noise from switching circuits. The screen itself has a switching LED backlight. The LEDs in your game are also being switched on/off by the LED controller.

Ground loop noise can be very difficult to track down and resolve. If it's possible to increase the switching frequency above audio frequencies that can help to "eliminate" the hum.

Short of making sure the speaker cables are out of the way and the ground cable is firmly attached, I'm not sure what else to try without spending some time thinking about it.

Unfortunately, the GI OCD transistors aren't fast enough to switch above audio frequencies. Depending on which firmware version you have, it's either switching at 250 Hz or 1kHz. The 1kHz would be more likely to be something you would hear, so if you have v5 firmware on the GI OCD, we could try reprogramming it to v3. Feel free to email me if you continue having problems with it.

Make sure the speaker wire is not getting smashed by the ColorDMD. I had that problem once and had some problems.

Does disconnecting the ribbon cable that runs to the ColorDMD change anything?

Are you using stock WPC audio and video boards?

Are all of the boards in the backbox screwed down reasonably tight? Are all connectors on tight?

Thanks for the update. Glad you found a fix!