The DFPlayerMini page and "wiki" have confusing information.
- Their wiring drawing and their code do not match.
- The diagram shows Rx/Tx as 11/10 (Uno/Nano) but their two example sketches show pins 5/4.
- The reference of Rx/Tx is according to the DFPlayerMini, not the microcontroller (11 goes to DF RX)
- The #ifdef/#else for SoftwareSerial creates a "not declared in this scope" with a Nano, needing
defined(ARDUINO_AVR_NANO) - I like this blog page for all the right information: https://www.dfrobot.com/blog-277.html
- definitely need external power (the web site is using Arduino power)