Help | MP3 module can't read data in ()

Arduino can play with the code below(changing to next or previous audio).
But it seems it can't read the data inside ().
ex. When I change the number in the volume(), nothing happened.

#include <DFRobotDFPlayerMini.h>
#include <SoftwareSerial.h>
#include "Arduino.h"

SoftwareSerial mySoftwareSerial(11, 10); // RX, TX
DFRobotDFPlayerMini myDFPlayer;
//void printDetail(uint8_t type, int value);

void setup()
{
  //----------------------------------------mp3
  mySoftwareSerial.begin(9600);
  Serial.begin(115200);

  Serial.println();
  Serial.println(F("DFRobot DFPlayer Mini Demo"));
  Serial.println(F("sensing.. (take 3~5 seconds)"));

  if (!myDFPlayer.begin(mySoftwareSerial)) {  //Use softwareSerial to communicate with mp3.
    Serial.println(F("Error!"));
    while (true) {
      delay(0); // Code to compatible with ESP8266 watch dog.
    }
  }
  Serial.println(F("DFPlayer Mini working."));

  myDFPlayer.volume(10);  //Set volume value. From 0 to 30

  //----------------------------------------gesture
  while (!Serial);

  if (!APDS.begin()) {
    Serial.println("Error!");
    Serial.println("Detecting gestures ...");
  }

}

void loop()
{
  if (APDS.gestureAvailable()) {
    int gesture = APDS.readGesture();

 if (gesture == GESTURE_LEFT) {
      Serial.println("LEFT ");
      myDFPlayer.next();
      delay(5000);

    }
    if (gesture == GESTURE_RIGHT) {
      Serial.println("RIGHT ");
      myDFPlayer.previous();
      delay(5000);
    }
  }
}

Also, I want to assign which audio to be played.
It can't read and work. I think it's the same problem: can't read the data in the ().

 if (gesture == GESTURE_LEFT) {
      Serial.println("LEFT ");
      myDFPlayer.play(1);
      delay(5000);

    }
    if (gesture == GESTURE_RIGHT) {
      Serial.println("RIGHT ");
      myDFPlayer.play(2);
      delay(5000);
    }

I'm not pretty sure if that's the real problem...hope to solve this soon.

Thanks for everyone's help...

My reference for code