That trim pot could be the likely culprit as well.
I suppose it could be, but the output of the potentiometer and ground always yields a voltage of 0 to 12v. Would this be enough to rule it out as the issue?
Very true about the trimpot, I will do some measurements and get back to you.
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I couldn’t say for sure, but ive had pots malfunction in a way that it seems that either puts out nothing or everything. Which seems to me to be what you are describing. However, I really don’t have much experience with the engineering just the tinkering. I managed to tinker obsessively with these things until they work. LOL. That’s how I caught on to the potentiometer issues among others with my DIY journey over the last year and a half. When I built this circuit the first time, I didn’t even have a proper power source. I used two 9 volt batteries, and it worked! 
Yeah I’m not the greatest at soldering… but according the multimeter there’s no continuity, so it should be ok.
I just did some testing on the trimpot, and it seems to output a consistent ~-9.9v, which should be ok?
Quick update, I tried changing the potentiometer and something very weird happened. I measured the difference between the TRI OUT and ground and it gave almost exactly 4v. However, after testing and the usual high pitched oscillation from before played, suddenly the amp spat a loud cracking sound (as if I was plugging in a guitar while the amp was still on). Now the voltage reads the usual ~1.2v from before and the high pitched noise is gone.
I tried returning to the original pot for the same thing to happen (it initially outputed 4v with the noise before returning to 1.2v and no noise). Any ideas as to why this might be happening? Should I maybe try replacing all the components as you have suggested? I am only hesitant as I want to rule out any other potentially fixable issues.
No, you should not expect anything useful from measuring the red dots. The control voltages should be measured from the other ends of the resistors as shown in the Annotated Simple 1V/Oct Oscillator Stripboard link I posted above. There are two control voltages, the CV Input one and the one produced by the tuning knob, each with its own 100k resistor. Measure them separately. The sum of these two voltages is what determines the pitch, and should be up around +9 V.
It’s also not very useful to stick a voltmeter on the triangle (or square, or ramp) outputs, either, because these are or should be oscillating at audio frequency. Better to use a scope to look at them, or lacking a scope use an audio probe:
That would tell you at least whether the 3340 is producing a tone that’s not getting to the output or not, which would help isolate the problem.
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I would definitely do a breadboard.