Luukas Hiltunen on the work:

It is apparent that the capacity of Jean Sibelius (1865-1957) to transcribe new music onto paper and then to be interpreted by musicians came to a standstill during the last twenty-five years of his life (1932-1957), a period which, even during the composer's lifetime, albeit shorter (1945–1957), was known as the ’Silence of Ainola’. Why this was the case is a mystery in some quarters, but several ideas have been proposed, both by individual musicians and scholars as well as by Santeri Levas (1899-1987), who worked as the secretary of the composer from 1938 until his death and published an esteemed biography in 1960. 

Possibly the most notable factor was the severe hand tremors which, in those years, made it increasingly difficult to the composer to write letters, read newspapers and, of course, that most essential of all, write music. In addition, the composer suffered over the last decades from cataracts, which forced him to acquire spectacles in the mid-1920s, although he never wore them in public. The hand tremor was to some extent hereditary, for which there is nowadays a diagnostic name, essential or kineric tremor (G25.0), but the composer's heavy use of alcohol, which, especially during the 1920s when he was composing the Seventh Symphony in C major, Op. 105 (1914–1924), almost verged on alcoholism, must also have had an influential role. The composer's cognitive abilities were, however, remarkably lucid and consistent right up to the end, leading to a profound reflection on the meaning of life, the nature of reality and the essence and future of music. Even after the "grand burnings", which his wife Aino (1872–1969) called the destruction of a large number of composition manuscripts in August 1945 in the fireplace of Ainola, he spoke, although was feeling relieved, of a "magnum opus in progress which must be finished before time runs out". That magnum opus was the Eighth Symphony, JS 190, a work that conductor Serge Koussevitzky (1874-1951) was due to premiere with the Boston Symphony Orchestra as the culmination of the Sibelius symphony cycle, but whose manuscript pages were burnt to ashes, after almost twenty years of struggle. Although larger sheet music paper was then obtained to Ainola in the hope that would spark inspiration and enable music writing, no new music emerged, only arrangements of already existing works.

The now-published symphony orchestra transcriptions of the Two Organ Pieces of Op. 111 (2018, rev. 2022 and 2024), Intrada Op. 111a (1925) and Surusoitto [Mournful Music] Op. 111b (1931), shed fascinating light on the possible musical content of the Eighth Symphony. The idea for these arrangements arose not only for these musical-historical facts, but also for a very personal reason – the music of Sibelius has been very close to my heart ever since I was a very young child. One was struck by the composer's music like a bolt of lightning when first heard it at the age of 4, followed by a visit to Ainola at the age of 5, where a purchased poster of the young Sibelius, in his twenties, still decorates the headboard of one’s bed. It is therefore self-evident that when an opportunity arose for me to transcribe Intrada and Surusoitto [Mournful Music] for a symphony orchestra, it seemed the most natural task to undertake.

Although stylistically Intrada is identical in almost all respects to the Seventh Symphony and the Prospero scene from The Tempest (1926), it does, however, have a brilliance and gloriousness that neither of the mentioned works can attain. Thus the work could well represent the conclusion of the Eighth Symphony's finale or the concluding section of the first movement before the uninterrupted transition to the slow movement Largo [information on the transition is based on Sibelius's letter of gratitude to his copyist Paul Voigt after the transcription of the Eighth Symphony’s first movement in September 1933 – it should be emphasized that unless otherwise stated, all references to the content and nature of the Eighth Symphony are my personal speculation]. While the Seventh Symphony is flowing like a stream spontaneously and relentlessly, Intrada, even though encompassing that same flowability, provides a single moment along that stream. Intrada is like a radiant being of light, which even the Prospero reference in the central section [the conquered duke, the antagonist], or the brief three-measure echo in piano of the solemn forte-chords before the recapitulation, cannot diminish. This radiance is then even reinforced at the end by the rising chords, as if ascending towards eternity.

On the contrary, the musical content of Surusoitto [Mournful Music] is a lullaby, which is destabilized by changing and overlapping time signatures and ear-splitting dissonances, making it downright macabre. Despite its brief duration, approximately five minutes, the work manages to capture, in a halting way, a huge number of distressing emotions and feelings – not even the ending, whose apparent calmness conceals sadness in its most poignant and pristine form, affords any relief. When he began work on the Eighth Symphony in the summer of 1928, Sibelius assumed in a newspaper interview that he would continue to renew himself as a composer. Listening to Surusoitto [Mournful Music], and particularly the now-first symphony orchestra setting, one can truly assert that Sibelius accomplished that – the composition is a timeless piece of contemporary music, as is Beethoven's (1770–1827) Große Fugue (1825), quoting here Igor Stravinsky (1882–1971) who described Beethoven's composition as such.

Together, these pieces create an exciting, reciprocal whole that could be described as a two-movement Sinfonietta, and I am confident Sibelius would have granted his blessings for this title entry.