hmmm.
then check your solder points again for a short. or post pics of your work and we can check it out.
the half clock board is working right? just the speaker hum is the only issue you're noticing?
the only other thing I know to do would be to work through each mod, disconnecting it from the other circuitry, and do process of elimination. i was thinking the clock signal was getting into your audio path somehow and you were hearing it as the hum. but maybe not. as a generator of the hum, i suppose.
the other kits, the parts themselves, wouldn't do this. poor soldering can cause all sorts of issues. also in the backlight installation, something could have been damaged on the front PCB or any number of things. if it were me, and i wanted to get this figured out quick and not worry about it again, i'd just go through each mod, disconnect it, and see if it fixed the issue. start with the clock, then backlight, then prosound.
a ton of people have used similar configurations in their DMGs and gotten no issue, so i'm sure its not a compatibility issue.
check the tutorials again, and make sure you hooked everything up right of course.
and to just ask.... are you positive the hum wasn't there to begin with?