MP3 Player

/*
 *https://arduspot.blogspot.in/p/mp3-player_6.html
 */

#include "SoftwareSerial.h"
SoftwareSerial mySerial(10, 11);
# define Start_Byte 0x7E
# define Version_Byte 0xFF
# define Command_Length 0x06
# define End_Byte 0xEF
# define Acknowledge 0x00 //Returns info with command 0x41 [0x01: info, 0x00: no info]
# define ACTIVATED LOW

int mus = A0;
int a,c;
int buttonNext = 2;
int buttonPause = 3;
int buttonPrevious = 4;
boolean isPlaying = false;

void setup ()
{
  pinMode(mus, INPUT);
  pinMode(buttonPause, INPUT);
  digitalWrite(buttonPause,HIGH);
  pinMode(buttonNext, INPUT);
  digitalWrite(buttonNext,HIGH);
  pinMode(buttonPrevious, INPUT);
  digitalWrite(buttonPrevious,HIGH);
  mySerial.begin (9600);
  delay(1000);
  playFirst();
  isPlaying = true;
}

void loop ()
{

  if(Serial.available()>0)
  {
    c = Serial.read();
    if(c==1)
    {
      playNext();
    }
    else if(c==2)
    {
      playPrevious();
    }
  }
  a = analogRead(mus);
  Serial.println(a);
 if (digitalRead(buttonPause) == ACTIVATED)
  {
    if(isPlaying)
    {
      pause();
      isPlaying = false;
    }
    else
    {
      isPlaying = true;
      play();
    }
   }

 if (digitalRead(buttonNext) == ACTIVATED)
 {
  if(isPlaying)
  {
    playNext();
   }
  }

 if (digitalRead(buttonPrevious) == ACTIVATED)
 {
  if(isPlaying)
  {
    playPrevious();
    }
  }
}

void playFirst()
{
  execute_CMD(0x3F, 0, 0);
  delay(500);
  setVolume(20);
  delay(500);
  execute_CMD(0x11,0,1);
  delay(500);
}

void pause()
{
  execute_CMD(0x0E,0,0);
  delay(500);
}

void play()
{
  execute_CMD(0x0D,0,1);
  delay(500);
}

void playNext()
{
  execute_CMD(0x01,0,1);
  delay(500);
}

void playPrevious()
{
  execute_CMD(0x02,0,1);
  delay(500);
}

void setVolume(int volume)
{
  execute_CMD(0x06, 0, volume); // Set the volume (0x00~0x30)
  delay(2000);
}

void execute_CMD(byte CMD, byte Par1, byte Par2)
// Excecute the command and parameters
{
// Calculate the checksum (2 bytes)
word checksum = -(Version_Byte + Command_Length + CMD + Acknowledge + Par1 + Par2);
// Build the command line
byte Command_line[10] = { Start_Byte, Version_Byte, Command_Length, CMD, Acknowledge,
Par1, Par2, highByte(checksum), lowByte(checksum), End_Byte};
//Send the command line to the module
for (byte k=0; k<10; k++)
{
mySerial.write( Command_line[k]);
}
}