An electret sound detector circuit provides a binary signal based on a preset threshold (adjustable with a pot). This is presented to the TM1638 as a “key” input via a 2N7000 mosfet. 

The ATTiny85 has a simple program that drives 4-digit LED 7-segment display via the TM1638 (a pair of “eyes”). It blinks (with some random fuzz in the blink algorithm) when sound is detected. 

All the details of the design and how it works are available in the project's GitHub Repo . Here's a little demo: