Why weird audio output from DFplayer mini?

Hi, I’m doing a simple audio output based on this link:http://www.rhydolabz.com/wiki/?p=17661
with a modified code as below. But the result is so weird that I only hear a short burst of noise (together with the red light from the DFplayer) i think every 20 seconds or more. The output of Serial monitor in 9600 rate is exactly this: ~⸮ ~⸮~⸮~⸮~⸮~⸮~⸮~⸮~⸮~⸮~⸮…
By the way the RX of the DFplayer is connected to pin 1 of my board, the TX to pin 0.
My goal is to play 1 file (15 second mp3 in a folder labeled mp3).

#include <SoftwareSerial.h>;
#include <DFPlayer_Mini_Mp3.h>;
 
void setup () {
   Serial.begin (9600);
   mp3_set_serial (Serial);    //set Serial for DFPlayer-mini mp3 module 
   delay(1);  //wait 1ms for mp3 module to set volume
   mp3_set_volume (25);
}
void loop ()
{
    mp3_play ();        //start play
    mp3_play (1);    //play "mp3/0005.mp3"
    delay (20000);
    mp3_stop ();
}

Where is the link to the library you are using?

You realize that loop() loops, right?

If you are using Serial to talk to the MP3 player, why the f**k are you interfering with that by opening the Serial Monitor app?