I will preface this with where my study comes from. In the study of the composer Wilhelm Furtwangler, this topic comes up. For a reference, I will recommend. The Devils Music Master by Sam Shirakawa as the only substantial English language biography of Furtwangler.

In the analysis of the recording of April 19th, 1942, David Benjamin Levy writes in Beethoven: The Ninth Symphony that Furtwangler was furious and angry with the Nazi regime for trying to tie something as pure as Beethovens Ninth to the occasion of Hitler's Birthday. The Nazis ignored the message in favor of the messenger, a German musician hailed as one of if not the most important composer. As a result, Furtwangler changes the tempo. Furtwanger had resisted but was forced, so, according to Levy, uses the symphony as a form of protest. You can hear and see the last few minutes of this legendary preformance here. As mentioned, the Germans didn't care for the message, Furtwangler made his own. The film Taking Sides, which focuses on the DeNazification process of the conductor offers its own theory about that night, ending the movie with the last few seconds of the fateful clip I linked, suggesting disgust with the Nazis after Furtwangler tries to clean the hand he shook with Goebbles with his sweat towel.

While this only answers the question partly, I would argue that the fateful recording says a lot about the German perception of Beethoven.

Like other aspects of the German cultural canon, the cultural authorities in the Third Reich tended to co-opt these figures for their own purposes and leave out any troubling details that would run counter to racialist ideology of National Socialism. This often meant exaggerating particular aspects of these figure's biographies and works to make them fit into the narrative of them as proto-National Socialists. Beethoven was a prime example of this process of selective appropriation.

Despite his Dutch roots and name, Third Reich-era musicologists would argue that Beethoven's intrinsic Germanness proved his heart and soul was truly German. In 1934, the journal Volk and Rasse would publish an article emphatically stating Beethoven had no tainted racial background and was a pure Germanic soul. Other musicologists would argue that Beethoven's antipathy towards Napoleon stemmed not from his dalliances with French Revolutionary ideology, but rather was a reflection of the German national wakening against a foreign invader. The process of claiming Beethoven for necessitated a highly selective reading of Beethoven's biography. Physical descriptions of his shortness and swarthy complexion were minimized and his deafness used to prove German racial superiority. One youth-targeted book on Beethoven noted that the composer created his best works when going deaf which was a sign of his Germanic spirit triumphing over adversity.

The notion of Beethoven as a heroic figure of his age long predated the Third Reich and was a cultural trope easily adaptable to the new regime. Beethoven emerged a titan of Germanic or Nordic genius in state media and among German musicologists who easily self-coordinated with the regime. Beethoven's music thus became both exclusionary and universal in the Third Reich's discourse. On one hand, materials produced on Beethoven stressed his singular Germanic genius was the gift of the German Volk to the entire world. This sentiment dovetailed with the wider German volkishc intellectual conceptualization of "universal" in which German philosophical and cultural ideas were taken as a the default for the highest humanity could achieve. Beethoven's energetic romanticism became so associated with National Socialism's own style of extreme nationalism that it became difficult to disentangle the two. The rhetoric surrounding Beethoven became highly exclusionary and often treated works of his non-German Romantic contemporaries as derivative and a pale reflection of the master.

The Ninth in the Third Reich became a victim of this entanglement of ideology and music. It was one of the most performed pieces in Beethoven's oeuvre during the Third Reich and was often played at prominent state events such as the opening ceremonies of the 1936 Olympics. The 1936 Olympic's instrumentalization of the Ninth was typical of the Third Reich's approach to the Ninth. Coming at the climax of a festival of Olympic Youth which blended a series of tableaux representing medieval and modern themes with some 10000 , the finale of "Heroic Struggle and Death Lament" featured mass dance routine of a sacrificial death for the fatherland, in which "Ode to Joy" would signal a rebirth accompanied by searchlights and a ring of fire around the stadium that created a dome of light. The message of the spectacle, whose musical elements were choreographed by Carl Orff and Werner Egk, was open: Germany's national regeneration under National Socialism was complete and sanctified by the works of the German masters. The call for universal brotherhood was still there in the music, but the performative context made it clear that brotherhood had a decidedly racialist and political tinge to it. To be a brother for the Third Reich's Beethoven, one had to be an Aryan.

But the performative and interpretative nature of music often was a double-edged sword for the Third Reich and Beethoven. The Austrian composer Hanns Eisler would claim "Ode to Joy" was an antifascist work and that the Third Reich's exclusionary definition of brotherhood was at odds with Beethoven's intent. In Vichy, the Germans encouraged the performance of Beethoven as part of a larger program of asserting German cultural dominance, but French conductors and musicologists stressed the nature of "Republican Beethoven" that was at odds with the volkisch Beethoven in the Third Reich. Critics of the Third Reich would often use the artistic heights of the German masters to contrast how low Germany had fallen under the leadership of Hitler. British and American wartime writings on German history would often invoke the spirit of Beethoven, Goethe, and Schiller to demonstrate how far the Germany of Hitler had departed from its roots.

One of the more notable instances of this reclaiming of German music for the Allies was the wartime performances of Toscanini. An emigre opponent of Mussolini who was once roughed up by Squadristi, Toscanini would repeatedly assert that the fascist regimes of Europe were antithetical to the cultural and artistic traditions they claimed to be the guardians of. As the music director of the the NBC Symphony Orchestra, Toscanini would often incorporate German and Italian works into special performances in support of the Allied cause. For example, in the Winter of 1942, Toscanini led the Metropolitan Opera of New York benefit concert for the American Red Cross in a production of Wagner. After Italy's surrender and the downfall of Mussolini in September 1943, Toscanini conducted a victory concert of both Beethoven's Fifth symphony and Rossini's "William Tell" (itself based on a Schiller poem) as well as "The Hymn of Garibaldi" and "The Star-spangled Banner." When approaching the Romantics, Toscanini would frame their music as reflective of the ideals of 1789 and 1848, in short, a liberal tradition explicitly rejected by the regimes of Hitler and Mussolini.

Toscanini's appropriation of Beethoven and the other Germans masters demonstrates the malleability of music for ideological purposes. Concurrent to Toscanini's efforts in New York, Goebbels arranged for a special performance of the Ninth for Hitler's birthday in 1942 and the Volkischer Beobachter proclaimed that at the conclusion of the choral performance,

the hymn resounds over all distant countries where German regiments stand guard, then we ant everyone...to be equally aware of the seriousness of the hour and to experience the tremendous happiness of being able to witness and take part in this, the greatest historical epoch of our Volk.

Furtwängler famously tinkered with the performance of the Ninth, but the Propaganda Ministry often used both the prominence of conductors like Furtwängler and von Karajan as cultural capital to perpetuate its own highly selective interpretation of German music.

Sources

Buch, Esteban. Beethoven's Ninth: a political history. Chicago: University of Chicago Press, 2003.

Dennis, David B. Beethoven in German Politics: 1870-1989. New Haven [u.a.]: Yale Univ. Press, 1996.

_. Inhumanities Nazi Interpretations of Western Culture. Cambridge: Cambridge University Press, 2012.