Well, of course you can always claim they are not NECESSARY. In the limit, a book is just a visual novel with no choices, images, animation, sound or any other media besides the text. And a good book story can be amazing and take you to heaven and hell.
But I always view Visual Novels as the perfect evolution of books, taking the best from any other media and encompass it to tell a beautiful story with a greater impact.
And while animations for me can almost always be taken out without any difference in the impact a story generates, music are extremely powerful in my opinion. Music really touch something in my subconscience.
For example, something a visual novel can do that a book can't is just put an image and the right music and you can totally understand what is going on. Sometimes two characters saying nothing to each other (no text) but the right music totally conveys they are feeling angry, happy, sad, in love.
Of course, I am not trying to convince you, just mentioning why for me music is important. I really wished that for any animation devs do, they would actually take time to set the right music for the scenes. I am absolutely certain, the overall effect would be huge.