An arduino-based MP3 player using the DFPlayer Mini and the OneButton, DFPlayerMini_Fast, FireTimer & EEPROM libraries. Be sure you have those installed before attempting to compile the sketch.
The sketch implements 3 buttons. Button one skips to the next / previous tracks with a single or double press respectively, and decreases the volume when held. Button 2 pauses and resumes playback when pressed, stops playback on a double click and increases the volume when held. Button 3 randomises playback when pressed, but is otherwise unused. You can have as many buttons as you have digital or analogue pins, and can assign actions to 5 buttons states (click, double click, long press start, long press hold and long press release).
It is possible to implement every function of the DFPlayer by sending discrete commands from your sketch, but in my opinion a good library makes things much neater and it doesn't add much weight to the code. FireTimer is used here to add a nice volume ramp delay as it is required by the DFPlayerMini_Fast library anyway. The built-in EEPROM library is used to store the current volume in the EEPROM. Tested on an Arduino Nano, there are plenty of resources left for extra code for example running RGB LEDs, an LCD display or even a touchscreen.