Guitar pedal effect voltage problem
Use your guitar lead and amplifier as a "stethoscope".


Turn your amplifier down but loud enough to hear the buzz when you touch the jack tip.
Connect the Big Muff ground to your amplifier ground.
Plug the guitar lead into the amp.
Use the other end as a touch probe.
Put an audio signal into the input. A steady oscillator signal is ideal. You should be able to find a suitable signal generator app for your phone.
Probe at the collector of each transistor starting at the left side of the circuit. Note that there will be a DC "thump" as you connect.


This should narrow down the loss of signal to one area of the board.

Proceed from there.