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Sory, if have no experienc with piezos. I'd use OP-Amps as amplifiers, maybe.
I think others can answer this better.

Normally, OP-Amps are used.
https://www.google.com/search?source=univ&tbm=isch&q=piezo+op-amp&client=firefox-b-d&sa=X&ved=2ahUKEwi90ouonpbpAhUD_aQKHaPnApAQsAR6BAgJEAE&biw=1609&bih=864

void loop() {
int piezo = analogRead(piezo);This works:confused:

You said this does happen with piezos only, not with buttons.
Do you use the same code for the buttons? If yes, still, the piezos are the problem. maybe voltages spikes?
Can you post the missing...

What means "a problem". Are they played backwards? ;)
AudioMemory(20);
seems a little low

Please add sourcecode that can be used. Remove the stars.

If it works with buttons, the problem is the connection with the piezo, not the code.