The Code:

/*
  Arduino Christmas Song
  
  Based on a project and code  by Dipto Pratyaksa

  Modified for Christmas 2023 by Meganano,  on Dec 15, 2023.
*/
#include "pitches.h"

#define melodyPin 3
#define pirPin 7
int pirVal;

// Jingle Bells
int melody[] = {
  NOTE_E5, NOTE_E5, NOTE_E5,
  NOTE_E5, NOTE_E5, NOTE_E5,
  NOTE_E5, NOTE_G5, NOTE_C5, NOTE_D5,
  NOTE_E5,
  NOTE_F5, NOTE_F5, NOTE_F5, NOTE_F5,
  NOTE_F5, NOTE_E5, NOTE_E5, NOTE_E5, NOTE_E5,
  NOTE_E5, NOTE_D5, NOTE_D5, NOTE_E5,
  NOTE_D5, NOTE_G5
};

int tempo[]  = {
  8, 8, 4,
  8, 8, 4,
  8, 8, 8, 8,
  2,
  8, 8, 8, 8,
  8,  8, 8, 16, 16,
  8, 8, 8, 8,
  4, 4
};

void setup(void) {
  Serial.begin(9600);
  pinMode(melodyPin, OUTPUT); // Buzzer
  pinMode(pirPin, INPUT);    // Sensor Pin
}

void sing() {
  // iterate over the  notes of the melody:
  Serial.println(" 'Jingle  Bells'");
  int size = sizeof(melody) / sizeof(int);
  for (int thisNote  = 0; thisNote < size; thisNote++) {
    int noteDuration = 1000 / tempo[thisNote];
    buzz(melodyPin, melody[thisNote], noteDuration);
    int pauseBetweenNotes = noteDuration * 1.30;
    delay(pauseBetweenNotes);
    // stop the tone playing:
    buzz(melodyPin, 0, noteDuration);
  }
}

void buzz(int targetPin, long frequency, long length) {
  long delayValue = 1000000 / frequency / 2; // calculate  the delay value between transitions
  long numCycles = frequency * length / 1000; // calculate the  number of cycles for proper timing
  for (long i = 0; i < numCycles; i++) { // for the calculated  length of time...
    digitalWrite(targetPin, HIGH); // write the buzzer pin  high to push out the diaphram
    delayMicroseconds(delayValue); // wait for  the calculated delay value
    digitalWrite(targetPin, LOW); // write the buzzer  pin low to pull back the diaphram
    delayMicroseconds(delayValue); // wait  again or the calculated delay value
  }
}

void  loop() {
  pirVal=digitalRead(pirPin);
  Serial.print("PIR Reading: ");
  Serial.println(pirVal);
  if (pirVal == 1){
    sing();
  }
}