Need install Arduino Library Add On
・DFPlayer library V2.0
・DFRobot_utility library
・TimerOne
https://playground.arduino.cc/Code/Timer1
https://code.google.com/archive/p/arduino-timerone/downloads
This is what Im takjing about. The first is a project of which I have the written sketch. No version seems to upload in arduino library. The last link is all versions.