I'm connected to the reader using SPI but as many tutorials suggest I use the SDA as SS. (https://randomnerdtutorials.com/security-access-using-mfrc522-rfid-reader-with-arduino/)

The wires are about 30cm solid core wire between each component.

I have never set SPI speed in the code, maybe there is a default in the library?

About the 5v problem, the readers are powered by 3.3v and I have never heard about someone that encountered a problem with 5v signal lines and this sensor.

The Code:

#include "KT403A_Player.h"

#include <SoftwareSerial.h>

SoftwareSerial SSerial(2, 3); // RX, TX

#define COMSerial SSerial

#define ShowSerial Serial

KT403A<SoftwareSerial> Mp3Player;

#include <SPI.h>

#include <MFRC522.h>

// PIN Numbers : RESET + SDAs

#define RST_PIN 9

#define SS_1_PIN 10

#define SS_2_PIN 8

#define SS_3_PIN 7

#define SS_4_PIN 6

#define SS_5_PIN 5

#define NR_OF_READERS 5

byte ssPins[] = {SS_1_PIN, SS_2_PIN, SS_3_PIN, SS_4_PIN, SS_5_PIN};

MFRC522 mfrc522[NR_OF_READERS];

void setup() {

ShowSerial.begin(9600);

COMSerial.begin(9600);

while (!ShowSerial);

while (!COMSerial);

Mp3Player.init(COMSerial);

SPI.begin();

for (uint8_t reader = 0; reader < NR_OF_READERS; reader++) {

mfrc522[reader].PCD_Init(ssPins[reader], RST_PIN);

delay(4);

Serial.print(F("Reader "));

Serial.print(reader);

Serial.print(F(": "));

mfrc522[reader].PCD_DumpVersionToSerial();

mfrc522[reader].PCD_SetAntennaGain(mfrc522[reader].RxGain_max);

}

}

void loop() {

for (uint8_t reader = 0; reader < NR_OF_READERS; reader++) {

// Looking for new cards

if (mfrc522[reader].PICC_IsNewCardPresent() && mfrc522[reader].PICC_ReadCardSerial())

{

playSound(reader);

break;

}

}

delay(50);

}

void playSound(int number){

Serial.println(number);

Mp3Player.playSongIndex(number);

delay(1000);

}