From about fortyfive years of experience, since I read Lord of the Rings (LotR), I agree with Tolkien's opinion that fantasy is essentially a literary form.
After I read LotR, I tried to repeat or extend the experience by seeking in other art forms. At that time, there was not much material (that I could access, anyway) but I eagerly looked at such posters and pictures I could discover - including those by Tolkien himself.
But I found almost all of them unsatisfactory; and of those I did like (such as the work of Pauline Baynes) I would not say that they added to my experience of the books, or increased its depth - it was more a matter of taking the edge off my hunger. Tolkien's own pictures are often very good, but not in the sense of amplifying what he had done in the books. None of them looked like the real places (or people).
I have always found the musical side unsatisfactory too. My search began with medieval and folk music; and while I did develop a taste for these forms, I could never find anything which I felt fitted into the world of LotR - nothing that could have been perfomed in that world. Since then this has not changed. I never find that any musical setting genuinely fits the world of the book. Although I really enjoy Howard Shore's music from the movies (and own CDs of both the soundtrack and orchestral suite) - this is quite separate from my experience of the books. Certainly I cannot imagine Shore's music actually being sung or played in Middle Earth. The same applies to the way that songs are performed in audiobooks, and audio dramatisations adapting the novels; they may be good, but never 'fit'.
As for the matter of visual dramatic adaptation itself - again it is different. When Middle Earth is visually depicted in a movie or drama, the primary and specific fantasy element is closed-off rather than deepened. The LotR movies are about as good as movies can be - but there is a great gulf between fantasy in movies, and literary fantasy.
Literary fantasy is capable of much greater depth and active-power than movies - because reading a fantasy is (potentially) a collaboration, while watching an movie is (mostly) a passive and immersive experience. Now, clearly many people read novels as substitute movies; and want to be 'drawn in' and pulled along'; they call a desirable novel a 'page turner' and are desperate to reach the end and know what happened.
But the best novels, and the best fantasy, is much more than that; which is why we always want to re-read the best work, and engage with it/ think about it rather than 'lose ourselves' in it. I am, of course, aware that there are many/ most people who never re-read - but there are many/ most people who simply 'consume' LotR; and at most have fantasies 'about' it, or 'based-on' it - rather than getting from it the special quality that fantasy offers. And there are people who have that kind of 'exploitative' relationship to all books.
The kind of ideal, active engagement I am talking of is almost sure to be personal and idiosyncratic; it can't be manufactured, and it must be based on a spontaneous affinity between the reader and the work (and its author). There are likely to be only a few books that evoke this kind of reading, for most readers - and the great bulk of our reading is on a lower level, and for lower motives.
But if we do have this relationship with a book - and I suppose it would be the ideal kind of relationship which both author and reader seek - then we perceive the basic unsatisfactoriness of other art forms, when it comes to the genre of fantasy.