Setting derived classes items through base class variable

class Music: public mediaInfo
{
   public:
       std::string strProducer;
       float fMinutes;

   void setProducer(std::string strMusicProducer);

   std::string getProducer() {return strProducer;}
};

//////////////////////////////////////////////////// //Switch statement within main

case 'F':
{
  std::string strMusicProducer;
  inputCharacter.erase(0,1);
  std::cout << "Enter Producer of music item: ";
  getline(std::cin, strMusicProducer);
  aMixedArray[nInItIncreaser]->setProducer(strMusicProducer);
}

///////////// //Creating new items

int init_mixed_array()
 {



for (int nInItIncreaser = 0; nInItIncreaser < MAX_INIT_ARRAY_SIZE; ++nInItIncreaser)
     {
       int nInItType = (nInItIncreaser / PER_GROUP);
       std:: cout << nInItType << std::endl;
        switch (nInItType)
          {
            case 0 :
              aMixedArray[nInItIncreaser] = new Book();
            break;

        case 1 :
          aMixedArray[nInItIncreaser] = new Music();
        break;

        case 2 :
          aMixedArray[nInItIncreaser] = new Video();
        break;

        case 3 :
          aMixedArray[nInItIncreaser] = new mediaInfo();
          break;

        default :
        std::cout << std::endl << "Error" << std::endl;
      }
 }//end of for loop
  return 0;

The mixed array is creating new books, music, media items, and videos. The problem i'm having is assigning the producer within the music derived class.