Reduce MCU frequency on ESP32 to 20Hz

Job ID: 31664081

Budget: $10 – $30 USD

Aim:
Battery powered audio recorder (Digital microphone INMP441 on I2S bus) using the ESP32.

I use this open source code and it works already pretty good:
https://github.com/atomic14/esp32_sdcard_audio

With it's full 240Mhz it uses way too much power. But i am not clever enough to modify the code to let the ESP32 run with only 20Hz (which hopefully should be fast enough for audio recording?).