Arduino Nano and a ton of LED strips...

You have not connected the output enable pin to anything, this is pin 13 and needs to be connected to ground.

You also have your code wrong:-

const int ShiftPWM_latchPin=10;
const int ShiftPWM_dataPin = 11;
const int ShiftPWM_clockPin = 12;

The way it is wired it should read:-

const int ShiftPWM_latchPin=11;
const int ShiftPWM_dataPin = 12;
const int ShiftPWM_clockPin = 10;

And you need decoupling capacitors.

Not really sure why you have diodes across your piezos.

To cut out the negative ringing.

Or 1M resistors.

To provide a load and so limit the peak voltage.

Oops, I must have overlooked the OE pin being tied to ground in the schematic, the etched PCB does have it grounded on each register. As to the code modification, thank you, that was a bit boneheaded of me... haha

If I'm correctly understanding what Mike has posted and what I've read on decoupling capacitors since, I will need one on each register's VCC pin and one on the 12v line coming from the PSU's 12v rail..? I just want to make sure before I make another trip to the parts store.

Paul: Are you saying the ground should run through the emitter pin and out the collector pin, the opposite of its current state?

Thank you both very much for looking it over, I was afraid I was way off somewhere and would end up having to de/re-solder all the components and fabricate a new board... Before Paul's suggestion was posted and it was noted I'd plotted out the etch with the pinout for MOSFETs instead of NPN for tip122s, I desoldered all 24 transistors and milled out the PCB holes to allow reversing the E/C pins to fit with the etch's mistaken use of the middle pin being the output/drain.

One on each chip is essential, the one on the 12V is less so but it can't do any harm. As drawn like Paul said the transistors have their emitters and collectors the wrong way round.

Another smart moment... The etch is actually correct, in that case, having the collector in the middle; I had the terms backward, as well (Gnd is -emitting- current to be -collected-, not collecting current to be emitted to x-output. I unknowingly described a capacitor instead.) Me = smrt. haha, I shall add the capacitors, re-solder the transistors and give it a shot again tomorrow or so. Thanks :-)

One last question comes to mind… Is it ok to run the post-capacitor 5v+ to the register’s MR pin from the VCC pin (as drawn), or should it have its own line jumpered from the 5v line pre-capacitor?

Yes it is fine either way round. Remember the capacitors must be as close to the chip power pins as possible.

*** UPDATE ***

After adding the decoupling capacitors, it works like a charm on 23/24 of the transistors. The top one ("R" of channel #4 [zero-based]) is outputting 0.06v on the collector pin, whereas the other transistors output .6v. The brightness is notably lower, but only on this particular one. So I swapped it out with a brand new TIP122, but the problem persists. I am getting ~2.4v readings on the other resistors between it and the base pin, but only .6v on this one pin. Other than this, it works as expected (thus far, I will further test it tomorrow or so).

The only code driving this, since I tested as I soldered in each group of 3 transistors, is: ShiftPWM.SetAll(255); delay(2000);.

Any idea as to what could cause this little oddity? I've once again thoroughly checked for shorts/bridges, etc. Thanks!