Faulty operational amplifier of electret mic preamplifier
Here's the circuit you linked to with a couple of red circles on it: -



Check that capacitor C1 isn't somehow leaking some DC current through to the -Vin pin.

Also check that R4 is fitted correctly and you can measure the same voltages on each of it's pins as you do on the op-amp pins. R4 could be broken - can you measure it with a meter?

Measure that copper tracks are fine between R4 and the op-amp.

If necessary lift C1 from the circuit to check that -Vin and +Vin become the same level.

The OPA345 is unquestionable a good device for this application if you don't care too much about noise. The AD8606 does the same but will produce less noise.