Do you have test gear? Scope, Signal generator, DMM, and a phantom power supply to power the microphone, cap tester?
http://recordinghacks.com/microphones/AKG-Acoustics/C-414-EB
Gain stage is simple
First stage has a bootstrapped drain look at the 3.3uf
The pair is like a Sziklai follower pair the NPN Vbe is fairly constant so the current in the 2.2k is fairly constant
Output stage is engaged for the two high pass selections and almost bypassed for no HPF
Does the sound change with the different highpass filter settings?
Check the transformer for a broken wire on the primary and secondary you can still get a weak signal with a broken wire
Could be a capsule so test to make sure what is bad.
Check for voltages at the transistors Gate, Drain, Source, Bases, Emitters and Collectors(check the 2nd NPN at the three HPF settings) . Check the left side of the 4.7megs with for DMM for the capsule voltages. Check the voltage at pins 2 and 3
I would remove the capsule wires from the circuit and cap couple a signal generator to the input of the circuit and scope the circuit nodes.
I would not assume it is the capacitors without testing.