Hi - If  anyone else comes across this thread ,  this seems to work:

void Reduce()

{  

 unsigned char Reduce[4] = {0xAA,0x15,0x00,0xBF};

   mySerial.write(Reduce,4);

   //Reduce the volume(15) Command: AA 15 00 BF

     delay(1000);

    play(0x01);//Play file0001 as test of volume

  }

void Increase()

{  

 unsigned char Increase[4] = {0xAA,0x14,0x00,0xBE};

   mySerial.write(Increase,4);

   //Increase the volume(14) Command: AA 14 00 BE

     delay(1000);

    play(0x01);//Play file0001 as test of volume

  }