Remove the mic and R1.Is there a simple way to remove the electret mic and replace it with my audio jack?
Add three resistors to attenuate the stereo line-level inputs and mix them into mono.
I used enamel-insulated wire that is 1mm thick to make my coils. I got the wire from a speaker's crossover coil. I spaced my coils away from the board and away from other parts and wires.