His schematics doesn’t match the stripboard layout, so not sure what his gate and CV does (*) (too lazy to decipher it right now). Also, the 100k pots look pretty low – most designs have 500k or even 1M there – but it’s pretty hard not to get any sound out of an APC if everything is wired up correctly and you mess with the knobs a bit. Probably some trivial build glitch. Check everyone one more time.
*) APC CV is usually connected to the 555’s control input, which messes with the internal voltage thresholds in slightly odd ways – “neutral” CV is 2/3 of the supply language, and you can dial the frequency up and down from there. Not sure what his gate does.
EDIT: Looks like GATE goes to the RESET pin, which is left floating if nothing’s plugged in. That may interfere with the function; the datasheet recommends pulling it high if not used (which is also how it’s done in the schematics), even if I don’t think it’ll completely block oscillation. You may want to add a pullup resistor between pin 4 and the supply rail on the upper 555 (the lower has a wire from 9V to pin 4 already).