I seem to recall that the subject of Rosemary Brown's been debated here before but I could be mistaken and I don't have time to check.
What surprises me is that a number of distinguished musicians, including composers Humphrey Searle (with whom I studied) and Richard Rodney Bennett, expressed the view that there was something in it. As j_menz has suggested here and many others have done elsewhere, if Brownesque effusions are all that major composers can manage in an "afterlife", then who needs afterlives? Schubert and Liszt are indeed excellent examples of composers who were forging new and interesting paths in their last years; Chopin, with his ever more sophisticated use of counterpoint is another who came to be under the Brown spotlight. Whilst I do not think for one moment that RB did the kind of thing that she did for fame, money or any other dubious motive, I do nevertheless believe that she was at best delusional in terms of what she though she was doing and how it supposedly came about.
That said, the notion of "finding" music in the air, so to speak - or somehow accessing it as though it was already there - is nevertheless a phenomenon less easy to dismiss. Stravinsky testified to it, as did Schönberg; Busoni referred to the rôle of the composer as being somewhat more akin to that of a diviner and illustrated this in his Fantasia Contrappuntistica, describing it as "compilata per il pianoforte da Ferruccio Busoni". Elgar famously claimed to have discovered some of his ideas when walking in the Malvern Hills, Worcestershire, England, on the grounds that they were already there, as if awaiting someone to tap into their resource; thee is no doubt that something of this kind was at work when Anthony Payne put together Elgar's Third Symphony from the composer's sketches, because his very considerable composerly abilities and scholarly knowledge and understanding of Elgar's music alone would simply not have been sufficient to enable him to bring to life the work that he has done. All of this might also sound fanciful to some, but a good deal less so, I submit, than the ramblings of Ms Brown. Considering this divination phenomenon prompted the third of my own Sieben Charakterstucke for piano, during the course of which Elgar effectively joins hands with Schönberg and Busoni in the guise of multiple and simultaneous allusions to their work.
Best,
Alistair