Electronic – Use of Cap here

For starters, you haven't decoupled the microphone power at all. You should split R1 into two resistors (e.g., 4700Ω each), and put a fairly sizable ceramic capacitor (0.1 to 10.0 µF; 1.0 µF gives you a cutoff frequency of 34 Hz) from the middle junction to ground. This is the most direct path by which power supply noise can get into the audio input chain.

The next way would be if the opamp has poor PSRR, in which case, you should decouple its supply with a dedicated resistor and capacitor. The problem here is that you don't have a lot of headroom to work with, which will limit the size of the resistor you can use (voltage drop based on the average current that the opamp requires), which in turn will require a relatively large capacitor in order to get a low cutoff frequency.

For a bargain in FET switches and precision analog resistor networks in the same package, take a look at MDACs like the AD7528. The signal path is just that : FET switches and resistors, nothing else.

They have been used in some quite high end audio products.

Downsides:

• Conversion from gain settings (dB, log scale) to DAC codes is easiest with a microprocessor.
• The 8-bit AD7528 only covers a 48dB gain range, with coarse steps at the bottom end of the range.
• There is some crosstalk between channels in the same package, careful design can mitigate that to an extent.

One design approach, mitigating the second and eliminating the third, is to use one AD7528 per audio channel. Use both its channels to extend the attenuation range, e.g. using one as a coarse (6dB steps) and the other as fine gain adjustment, probably with a buffer (emitter follower) between them.