How to shield your microphone the best way
EMI Ingress and suppression is far from trivial, and once understood where all your sources are from your geometry of conductors, the solution might appear simple.
There are a few common noise sources.

Line hum (often modulated SMPS PWM carrier noise from chargers or PSU)
cell phone pings to server
Microwave Oven line hum (modulated carrier)
SMPS modulation on charger due to load variations
ground shift noise from all ground wires external to cabinet acting as antenna to conduct and radiate shared noise being radiated.
AM, HAM radio noise.

You will notice some mic's have large ferrite clamps to raise the common mode impedance of signal and ground so that a small shunt capacitor (1nF) on the analog output shunts the difference signal.  This only helps a little bit for RF on high impedance electrets.
ALL (!) VGA video cables use molded ferrite clamps but these are 75 Ohm signals and this works well for 2-way ingress/egress.
The problem is that Electric Mic's are high impedance open drain buffers that require a drain pullup resistor for gain AND DC bias. Meanwhile due to self-capacitance between self-inductance and conductance of ceramic ferrite mixtures, you rarely get more than 1 KOhm at useful frequencies from 10MHz to 1GHz. These are specified for different ferrite core materials that it works best on low impedance sources like 50 Ohms and not 10K drain pullup and 0V ground as a lossy low pass filter with the shunt caps to ground as a common mode Pi filter for LCL filter.
Rejecting the common mode noise requires *balanced wires (twisted pairs), separate ground shield to drain noise currents or high quality coax with a ceramic load cap added to attenuate the drain pullup R at 15kHz for -3dB.
This tutorial explains some of the details.
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
Recommendations

add 1nF across mic jack inside USB amp.
Ensure mic wires are either coax or twisted pairs with separate gnd shield.
Verify noise source by spectral characteristics ( don't assume)
Add many turns of mic wire around suitable ferrite torroid
Shield USB amp choosing common ground carefully.
use your hand around foil to various sources and sinks to find optimal shunt path.