Can't use both sensors via NewPing and DFPlayer mini

I want to use 2 distance sensors and a DFPlayer Mini to play MP3 files when a distance is crossed.

Everything in the serial goes weird and nothing works if I combine the code. Separating the MP3 code and sensor code does work.

It should countdown from 8 to 1 and then print text. This does work without the DFPLayer.

if I comment the ‘myDFPlayer.play(1);’ the countdown does work
if its uncommented the countdown and player both break
I was trying around with different pins, maybe it was caused by timers, but that didn’t help

//Include the NewPing library
#include <NewPing.h>
#include "Arduino.h"
#include "SoftwareSerial.h"
#include "DFRobotDFPlayerMini.h"

SoftwareSerial mySoftwareSerial(3,11); // RX, TX
DFRobotDFPlayerMini myDFPlayer;
void printDetail(uint8_t type, int value);

//int frontPin = 13;
//int underPin = 12;

int maxCount = 8;
int countIndex = maxCount;
int lastTime = 0;

int maxWaitingTime = 1000;
int waitingTime = maxWaitingTime;

bool spotted = false;
int frontDist = 0;
int underDist = 0;
bool passedDoor = false;

int falseFrames = 0;
int maxFalseFrames = 2000;

// This simple code allow you to send data from Arduino to Unity3D.

// uncomment "NATIVE_USB" if you're using ARM CPU (Arduino DUE, Arduino M0, ..)
//#define NATIVE_USB

// uncomment "SERIAL_USB" if you're using non ARM CPU (Arduino Uno, Arduino Mega, ..)
#define SERIAL_USB


//Make new sonar sensors
NewPing underSonar(5,6);
NewPing frontSonar(10,9);

void setup() {
  // put your setup code here, to run once:
  
  Serial.begin(9600);

}

void loop() {

  //Make a variable and assign a value to it
  underDist = underSonar.ping_cm();
  frontDist = frontSonar.ping_cm();
  
  
  //spotted = digitalRead(frontPin);
  //under = digitalRead(underPin);

  if(underDist < 100 && underDist > 0){
    passedDoor = true;
  }

  //Serial.println("front: " + front);
  //Serial.println("under: " + under);

  // check if a person is detected in front
  if (frontDist < 100 && frontDist > 10) {

    falseFrames = 0;
      
    int currTime = millis();
    
      if(currTime >= lastTime + waitingTime){
         Serial.println(countIndex);
         myDFPlayer.play(1);
         
  
         countIndex = countIndex - 1;
  
         lastTime = currTime;
         waitingTime = waitingTime - 100;
  
         if(passedDoor == true){
            Serial.println("HAPPY RAMSAY");
            Reset();
         }
  
         if(countIndex < 0){
          if(passedDoor == false){
            Serial.println("MEAN RAMSAY");
          }        
          Reset();
         }
    }
  }else{

    falseFrames = falseFrames + 1;

    if(falseFrames > maxFalseFrames){
      falseFrames = 0;
      Reset();

      Serial.println("reset falseFrames");
    }
    
  }

  //delay(100);
}

void Reset(){
  //Serial.println("resetting count down");
  countIndex = maxCount;
  waitingTime = maxWaitingTime;

  passedDoor = false;
}

If you need any more info let me know

I'd start by ditching the DFRobot library. Install the DFPlayer Mini MP3 by Makuna and get better results.