Good point, and the book talks about this on page 177, right before section 4.14.3. I should add to the slides since it differs from sounds and sprites....
Since the music items in the array cannot be scooted over as you indicate, just set the label to empty ("") - that way it won't be found again.
this -> music[y].setLabel("");
And, of course, you also then don't want to decrement the music count, either.