Using a piezo disc as an input and output device to ATtiny85
Add a decoupling cap (100nF) across the AVR's power pins, with leads as short as possible to the ciontroller;
For the zener diode use a voltage close, preferably slightly lower to the battery voltage. The trigger threshold of the input is ½ Vcc, so you definitely want the zener voltage higher than that;
Did you actually check the sound output volume level with the 10k resistor?
You probably want to turn the PWM pin in INPUT mode (high impedance) while sensing the input voltage, to not unnecessarily load the already weak piezo signal;
As you need to turn the output pin into input mode for sensing anyway, you can probably use just a single GPIO pin for both functions (saves parasitic capacity and leakage current).