(keywords: bad sound, noisy sound, power supply, squawking)
I spend several hours trying to make the DFPlayer Mini MP3 player work in the simplest configuration on the a breadboard (no Arduino).
I got erratic behaviour and sometimes the chip just looked dead.
Fix: Use a bench power supply
I was using an Elegoo Power conditioning circuit on my breadboard that was powered by a 9V battery. The MP3 player drew too much current, the voltage dropped and then it zombied. The solution was to replace the 9V battery with a mains power supply. Everything worked well then.
I expect that this will also be necessary when using it with the Arduino.