What could this be? I checked all solder joints, reseated the ATMEGA. Using a 9V PSU. Upon turning it on I just hear a ‘plock’ sound.
Voltage readings (according to the Power Distribution chapter in Troubleshooting):
1: 8.4V
2: 7.6V
3: -7.3V
Red points: all 4.9V
Green points: all -4.9V
Purple points: all 4V
Cyan point: -4.1V
Triplechecked the LED polarities 
Could you post photos of your board?
Sure! Sorry for the bad quality. Dark around here. Will post better quality pictures tomorrow morning.
PS: during construction the only doubts I had was about the orientation of the resistor network. Am I correct that the lettering should be placed inwards?
Resistor network looks good.
OK, thanks, good to know!
Fresh pics are here
I have a spare 328 lying around… would it be useful to try swapping it around or is this a useless maneuver? I would expect the leds to work with just the AVR, correct?
Yes, that could be a good test. Do you have anything to program it?
It already has a bootloader, Optiboot (since it came out of a broken JeeLabs LedNode). So I’ll just pop it into an Arduino and… oh wait! I see that you have your own bootloader, correct?
http://mutable-instruments.net/static/firmware/anu_bootloader.hex
Then load the firmware itself using MIDI?
http://mutable-instruments.net/static/firmware/anu_0.92.syx
OK… I think I remember you can use an Arduino as an ISP programmer… I’ll read up and go that route then
For this test you don’t really care which bootloader is there…
So use the chip with optiboot. Flash it with this .hex file:
http://mutable-instruments.net/static/firmware/anu_0.92.hex
Put it on your board. If it works, I’ll send you a replacement 328p with the correct stuff installed on it.
Well, that is interesting! Flashing went OK (328 in Duemilanove, avrdude to upload the .hex). Now: sound! But no LEDs. Controllers seem to work, but not the buttons. I get an oscillator, can use the knobs on the synth section. Sub oscillator/filter/LFO all seems fine (and cool too) But I can’t switch to the kbd/ctrl or drums part.
Fried multiplexer? A short somewhere? Hmm.
Hmm! Looking at the schematics. I think I’ve switched the 74HC165AN and M74HC59581 around. D’oh! Let’s reseat them.
Perfect, it works! And it’s a lot of fun I’ll take a look at the original 328 again. But no need to send me a new one, this one was doing nothing anyway
The original 328 has no issues whatsoever. I guess it’s a fluke that it didn’t make audio with the ICs switched around yet the other time it did make audio.
Thanks a lot, will post a pic with beautiful case in the consensus thread soon now to put on the knobs 
So glad you solved it! I looked at your pictures for ICs with the wrong orientation, but didn’t check for swapped parts…
With the swapped 74hc595/74hc165 + the official MIDI bootloader, the synth stays stuck in firmware upgrade mode. Having the wrong IC in place of the 74hc165 simulates a key press during boot which locks the synth in firmware upgrade mode. That’s why it didn’t boot at all with the original chip; but still booted with the optiboot bootloader.
Hah! Mystery solved
I put colored LEDs in mine (blue at the top, white/red/green below), but they’re a bit too bright will increase the resistance a bit.
Cheers, it’s a lovely piece of kit!