$( document ).ready(function() {
var refcode={
      Guitars     : "GuitarsMag",
      Keyboards   : "KeyboardMag",
      Drums       : "DrumMag",
      Microphones : "MusicMag"
}
$("#productsMusical").change(function(e){
      $('[name="referralCode"]').val(  refcode[ $(this).val() ] )
})
});

<input name="referralCode" value="" id="referralCode" type="hidden">

<select id="productsMusical" name="productsMusical">
  <option value="" selected="selected">--Please Select--</option>
  <option value="Guitars - Acoustic">Guitars - Acoustic</option>
  <option value="Guitars - Electric">Guitars - Electric</option>
  <option value="Guitars - Bass">Guitars - Bass</option>
  <option value="Keyboards - Acoustic Piano">Keyboards - Acoustic Piano</option>
  <option value="Keyboards - Digital Piano">Keyboards - Digital Piano</option>
  <option value="Keyboards - Synth">Keyboards - Synth</option>
  <option value="Drums - Acoustic">Drums - Acoustic</option>
  <option value="Drums - Electric">Drums - Electric</option>
  <option value="Drums - Band">Drums - Band</option>
  <option value="Microphones - Vocal">Microphones - Vocal</option>
  <option value="Microphones - Vocal Headset">Microphones - Vocal Headset</option>
  <option value="Microphones - Instrument">Microphones - Instrument</option>
</select>