Electronic – Replacing a condenser mic with a MEMS mic

Using a pot as a voltage divider as you do is not a good volume/gain control. The problem is that it makes your output impedance dependent on the gain. This has the effect of making your frequency response vary with gain as that output impedance interacts with the capacitance/impedance of your cable and whatever you are feeding this output into. You want the output impedance to be as low as possible. Do this by deleting R5 and R6.

To do the gain control without impacting the output impedance use a pot to replace either R3 or R4 (or both).

The nice thing about this type of gain control is that it actually controls the gain. With your configuration if the opamp was clipping and you turned the gain down you would still be clipping the opamp. But with the modified gain control, if you turn down the gain the opamp will stop clipping.

Do not remove C2, since it blocks the DC signals to your sound card. You might even consider increasing the size of C2 to as high as 68 uF. The larger C2 is, the better your low frequency response will be. 10 uF is the lowest I would go, but 22, 33, 47, or 68 uF are better choices.

Line level output (.9Vpp) is standard (https://en.wikipedia.org/wiki/Line_level), but I have found that the output level on phones can vary across manufacturers. Line level is probably your best starting point though.

A voltage divider (2 resistors) would attenuate the level coming from the phone if you need it - though you also have the volume control buttons on the phone to contend with.

Another item you need to consider is the DC bias voltage coming from your module. From the schematic, it looks like a 2 pin ECM was connected. In this case, the DC bias is on the Mic + line, with the AC voltage coming back to the module from the transducer. Since you're replacing the ECM with a phone output, you should be sure to remove the DC component from that Mic + line. Depending on where the ADC is internally in your module, removing Rx might not help (you should measure to double check). If not, you may need to add a series capacitor to the Mic + line for DC blocking.