The tone control stage is arranged around the next opamp, IC1b. Here the ratio R16/ R15 sets the gain at about 18 dB. The effect of R14-C2 is, in principle, the same as R4 and C7: a smaller value of C2 increases the lower cutoff frequency. The real tone control however it the RC network inserted between IC1a and IC1b. Potentiometer P1 sets the bass level, and P2 the treble level. The two capacitors in the network, C15 and C3, behave as frequency-dependent resistances for alternating voltages.
 
The tone control stage is followed by a small power amplifier based on the venerable LM386 in standard configuration complete with Boucherot network C13-R17 to dampen out the effects of loudspeaker impedance fluctuations that may cause motorboating and other forms of instability. Output power will be of the order of a few hundred milliwatts driving a small 8-ohm loudspeaker (kept well out of the microphone’s vicinity).

  The LM386’s gain is internally set to 20 to keep external part count low, but the addition of external parts R18-C9 between pins 1 and 8 allows the gain to be set to 20, 50 or 200. Without R18 and C9 the LM386’s amplification is 20, which is recommended here. For an amplification of 200, fit capacitor C9 and replace R18 with a wire. With both R18 and C9 in place the amplification is about 50. As a good alternative to a dedicated power supply, and in good 201x fashion the amplifier is powered over a USB-B cable via K1, with interference suppression and some buffering afforded by C11 and C16 respectively.