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With the audio probe I'm getting sound until the MEAT potentiometer on one side, and up and to R21 on the other. No more sound starting from C24 or R22/R23

Wait a minute...
I replaced the 2N5457 earlier with a BF245A as I read this should be able to replace it (and I couldn't find a supplier for a 2N5457 nearby)
Turns out the BF245A has a reverse DSG orientation compared to the 2N5457? I inserted it reversed and now the potentiometers are working

Q4 still measures 0 where it shouldn't, but all the rest seems to be working. I'm gonna see how it performs in a box

Thanks for following up so swiftly!

I reflowed all suspect joints and got a new reading on pin 7 of the 4558: 3.39

Q4 & Q1 still same readings, with too much ground...
SO, I took out a transistor tester and the transistors themselves are reading correctly. I suspected the socket and took it out for Q4 and wired the transistor in, instead. Still exact same readings.

As for the Lowfrequency generating IC I made a mistake in following up the pin numbers, so 19 and 12 should be swapped (my apologies). 19 is thus reading 2.82 and 12 is reading 0. This reading should be correct according to this image indicating the chip's grounds: 
So I'll assume this IC is seated correctly.

What could still be causing the grounds on Q1 & 4? I'll go through with an audioprobe later today to see if anything's changed so far.

Ty for the response, Scruffie!

I got these readings on the trannies: (P1,2,3):
Q1 2.37, 7.79, 0
Q2 7.79; 2.19, 3.43
Q3 7.79, 3.03, 3.73
Q4 7.79, 0 & 0
Q5 7.79  4.4, 4.05

This on the lowfrequency IC:
1: 1.58
2: 0
3: 4.1
4: 4.71
5: 4.72
6: 3.42
7: 3.42
8:3.45
9: 7.8
10: 7.02

11: 2.6
12: 2.81
13: 2.89
14: 1.35
15: 2.81
16: 2.83
17: 2.8
18: 3.44
19: 0
20: 2.83

This on the 4558 IC:
1: 1.53
2: 3.3 (fluctuating down?)
3: 3.3 (fluctuating down?)
4: 0
5: 3.44
6: 3.45
7: 0
8: 7.8

Thanks a ton already!

Another update:
I have built a tester kit as was explained on top of this forum, great stuff!

I have also replaced Q1 since I was starting to get problems there (so, earlier in the circuit, around what should be a buffer by JFET) but now I'm still losing signal after at C7 (nothing coming in there) and nothing at C23 or even R2, which still leads me to suspect the Q1 but I just replaced that with a JFET I know is working. I can only get an audio probe reading on the 1 of the Q1 though.

Should I replace the socket for this Q1 perhaps?

Thanks for the responses so far! Been learning a great deal, appreciate it tons!

Also measuring audio signal on the 3 Bpots now! (Except on the level/LBS pot, nothing on any of the legs; could the problem be with the pot itself?; Its resistance was measured correctly)

EDIT: This appeared to be intermittent, as a minute later I couldn't reproduce audio from the same poles. Actually measuring problems starting from Q2 now. How could I figure out what is shorting?
Am planning to build a tester kit to have a more reliable out of box testing environment next week when parts arrive!

I'll try to get some better pictures of around the problem area (according to audio probe) and the pots!
I've suspected the pots in some previous rounds of troubleshooting and always checked carefully that they aren't touching ground or any other components, as far as I can tell here, they're fine. And the audio probe shows that the sound signal drops before they reach the pots even.

I replaced the 4558 IC and the main problem persists. Pin 13 on the large IC gives sound though, for some reason?
ALL PINS on the 4558 now give audio though, which was previously not the case!
Will keep posted!

Thanks for the response!

I have ordered some new ICs and in the meantime spent some more hours trying to find other problems. The IC in question does not appear to show any shorts when measuring with the multimeter; and the pin 6 on the IC (4558) gives me audio with the probe (which is weird because the C9/R22/R23 that should be before it in the circuit do not give any sound when probed... Am I overlooking something here? Should some parts of the circuit liek R22/R23 not be giving audio with

I have also replaced the C9 (which was a 22nf100J) with a 22nf100K which has a 5% higher tolerance. Can I leave this like this or is the 5% tolerance essential for the functioning in this circuit?

(1) Project Name: Soy Sauce (lectric FX)
(2) General description of the problem: no sound when FX is on; bypass works & LED works.
(3) Steps that you have taken to try to resolve the issue: checked all solder joints, checked for bridges, doublechecked component position, measured wiring for continuity, created an audio probe and got signal up to C23/R21 (drops from these components on in the signal BUT I get signal on the MEAT and RUMP potentiometer)
Checked this cap and resistor: they seem to be the right value and measured correctly as well, so this is strange to me.

(4) List any substitutions you used for parts/values: used parts from the Musikding (German) kit

I have been troubleshooting this baby a bit too long for a few days on end so am looking for some extra eyes. I have learned a tremendous deal from this so far (my previous kits were BYOC clones years ago, so I have finally been learning to read schematics through this troubleshooting process and understand more what's going on under the hood!)

I might have, during the troubleshooting process, reversed the small IC for one test. Could this have damaged it? Voltages on it seem fine. If the IC is damaged; would all pins stop working or just 1 or 2?

I have also been doubting the positioning/orientation of the potentiometers as they are onboard ones; though the 1-pin is in the square hole so that should be correct iirc?

I have added some photos for inspection here: https://drive.google.com/open?id=1pRvwyFOShJH9QqT-WPtGUkAMTbPz-ImH

Thanks beforehand for anyone willing to take a look!
Kind regards
Matthias