Need to exit this loop, to then be ready to start again.

Sorry Robin, been ill.
This is the original code you put on, just added the serial print fadeout mp3

/*
   Initial Author: ryand1011 (https://github.com/ryand1011)

   Reads data written by a program such as "rfid_write_personal_data.ino"

   See: https://github.com/miguelbalboa/rfid/tree/master/examples/rfid_write_personal_data

   Uses MIFARE RFID card using RFID-RC522 reader
   Uses MFRC522 - Library
   -----------------------------------------------------------------------------------------
               MFRC522      Arduino       Arduino   Arduino    Arduino          Arduino
               Reader/PCD   Uno/101       Mega      Nano v3    Leonardo/Micro   Pro Micro
   Signal      Pin          Pin           Pin       Pin        Pin              Pin
   -----------------------------------------------------------------------------------------
   RST/Reset   RST          9             5         D9         RESET/ICSP-5     RST
   SPI SS      SDA(SS)      10            53        D10        10               10
   SPI MOSI    MOSI         11 / ICSP-4   51        D11        ICSP-4           16
   SPI MISO    MISO         12 / ICSP-1   50        D12        ICSP-1           14
   SPI SCK     SCK          13 / ICSP-3   52        D13        ICSP-3           15
*/

#include <SPI.h>
#include <MFRC522.h>

#define RST_PIN         9           // Configurable, see typical pin layout above
#define SS_PIN          10          // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);   // Create MFRC522 instance

//*****************************************************************************************//
void setup() {
  Serial.begin(9600);                                           // Initialize serial communications with the PC
  SPI.begin();                                                  // Init SPI bus
  mfrc522.PCD_Init();                                              // Init MFRC522 card
}
void PrintHex(uint8_t *data, uint8_t length) // prints 8-bit data in hex with leading zeroes
{
  char tmp[16];
  for (int i = 0; i < length; i++) {
    sprintf(tmp, "0x%.2X", data[i]);
    Serial.print(tmp); Serial.print(" ");
  }
}
uint8_t buf[10] = {};
MFRC522::Uid id;
MFRC522::Uid id2;
bool is_card_present = false;
//*****************************************************************************************//

void cpid(MFRC522::Uid *id) {
  memset(id, 0, sizeof(MFRC522::Uid));
  memcpy(id->uidByte, mfrc522.uid.uidByte, mfrc522.uid.size);
  id->size = mfrc522.uid.size;
  id->sak = mfrc522.uid.sak;
}

bool cmpid(MFRC522::Uid *id1, MFRC522::Uid *id2) {
  return memcmp(id1, id2, sizeof(MFRC522::Uid));
}

void deregister_card() {
  is_card_present = false;
  memset(&id, 0, sizeof(id));
}
uint8_t control = 0x00;


void loop() {

  MFRC522::MIFARE_Key key;
  for (byte i = 0; i < 6; i++) {
    key.keyByte[i] = 0xFF;
  }
  MFRC522::StatusCode status;

  //-------------------------------------------

  // Look for new cards
  if ( !mfrc522.PICC_IsNewCardPresent()) {
    fadeOutMP3();
  }
  else {
    if ( !mfrc522.PICC_ReadCardSerial()) {
      // do nothing;
    }
    else {
      playMP3();
    }
  }
  Serial.println("CardRemoved");
  delay(500); //change value if you want to read cards faster

  mfrc522.PICC_HaltA();
  mfrc522.PCD_StopCrypto1();
}
//*****************************************************************************************//a

void playMP3() {
  //PrintHex(id.uidByte, id.size);
  //Serial.println("hello");
  bool result = true;
  uint8_t buf_len = 4;
  cpid(&id);
  Serial.print("NewCard ");
  PrintHex(id.uidByte, id.size);
  Serial.println("");
  while (true) {
    control = 0;
    for (int i = 0; i < 3; i++) {
      if (!mfrc522.PICC_IsNewCardPresent()) {
        if (mfrc522.PICC_ReadCardSerial()) {
          //Serial.print('a');
          control |= 0x16;
        }
        if (mfrc522.PICC_ReadCardSerial()) {
          //Serial.print('b');
          control |= 0x16;
        }
        //Serial.print('c');
        control += 0x1;
      }
      //Serial.print('d');
      control += 0x4;
    }

    //Serial.println(control);
    if (control == 13 || control == 14) {
      //card is still there
    }
    else {
      break;
    }
  }
}

//==========

void fadeOutMP3() {
  // don't know what should go here
  Serial.println("fadeoutMP3");
  delay(500); //change value if you want to read cards faster
}

out come

fadeoutMP3
CardRemoved
NewCard 0x0A 0x97 0xF3 0x16 
CardRemoved
fadeoutMP3
CardRemoved
fadeoutMP3
CardRemoved
NewCard 0x8C 0xDE 0x63 0xA9 
CardRemoved
fadeoutMP3
CardRemoved
NewCard 0x3A 0x5F 0xDE 0x16 
CardRemoved
fadeoutMP3
CardRemoved

Also recognises the same tag if i put it to the reader twice.
So this is good.

fadeoutMP3
CardRemoved
fadeoutMP3
CardRemoved
NewCard 0x3A 0x5F 0xDE 0x16 
CardRemoved
fadeoutMP3
CardRemoved
NewCard 0x3A 0x5F 0xDE 0x16 
CardRemoved
fadeoutMP3
CardRemoved
NewCard 0x3A 0x5F 0xDE 0x16 
CardRemoved
fadeoutMP3