The first iteration of the piezo diaphragm microphone circuit includes:

• piezo diaphragm used as a microphone (represented as the AC source and 15nF capacitor)
• high pass filter with 250Hz cutoff
• 40dB amplifier
• low pass filter with 1kHz cutoff
• half-wave rectifier with a smoothing capacitor

The microcontroller will wake up at 2-4 second intervals, measure voltage at the rectifier, and write the analog voltage reading and RTC timestamp to the SD card. 

Parts are on order and should arrive in 30 to 50 days.