Sorry for disappearing for 10 days. I got tied up and then did a bunch of work without reporting anything.
Ultimately I wasn't able to tame the buzz/hum by recapping the DE sound board or adding 15000 uF (or more) smoothing to the 12 volt lines.
But as someone reported, I was able to solve the problem 99% with an external power supply. I feel this is a bit of a hack, but I'm tired of messing with this so I've wired it up.
If you hear constant hum that isn't the DMD or the lights, you can remove the hum for $15 and some of your time. To see if this scenario matches yours, disconnect the power from the DMD and go into diags. That will eliminate DMD hum and light hum.
In my case I was getting about 58 dB of hum. Now I'm getting about 34 dB. That's 4x less buzz/hum and pretty quiet. Also, there is no need to float the sound board (although that never really helped me).
I picked up the cheapest ATX power supply they had at Microcenter for $15. You only need 4 wires from the power supply output. Splice into those and make the right 6-pin connector for the sound board. If I was more patient, I'd order the appropriate molex connector and make an adaptor rather than splicing, but hey...
You also need to connect one of the pins to ground to make the ATX power supply turn on with power.
Keep in mind that the AC outlet inside the pin (JP in my case) isn't switched. By this I mean it always has power. To get around this, I spliced into the AC output that goes to the coin door (for the dollar bill adaptor). This was the only "permanent" change I made to the pin --- Although I could revert it fairly easily.
Here is the new power cable going to the sound board:
Here is the splice from the ATX power supply:
Here is how the switched AC goes to the ATX:
Note I took this last pic before mounting the ATX more solidly to the cabinet and connecting it to the existing ground braid. I've mounted the ATX on the side so that any screws or other stuff that potentially fall down don't make their way into the fan or ATX supply innards.
(I'll eventually add this to my web site, but I've not been good about updates in the past 6 months...)