Here's a modification to make a tone selector switch for the FM radio station's electret microphone output. Changes for actual use require adjusting the 8.2K ohm resistor and the 1nF capacitor based on the voltage supply.
A tone selector switch is used as a high pass filter to provide some tone adjustment. The 47nF setting reduces the low frequency and puts more of the midrange into useful talk power.
For FM, a 1uF setting is used which provides more full range audio suited to FM voice.
The electret mic element inherently has a strong gain. The intensity of voice and the position of person speaking can be adjusted to provide the best gain without distortion. Experiment with distances and component values to find the best results.
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-controls.html
FM Radio Station Part 5 Index
http://humanoidolabs.blogspot.tw/2013/12/fm-radio-station-part-5-index.html
