The Brahms Viola Sonatas were transcribed from clarinet after insistence from the publisher and most likely Brahms was looking for some extra money. What resulted was probably the first full-fledged sonata form for the instrument, and they are both a mainstay in viola repertoire. A really terrific historical (and formal) analysis of the work is done by James Creitz of which I’ll excerpt a bit here, but you can go check out the article here
Brahms originally composed the Two Sonatas, Op. 120 for his clarinetist friend Richard Mühlfeld of Meiningen, to whom he had previously dedicated the Clarinet Trio and Clarinet Quintet. After finishing the two sonatas, Brahms was requested by the publisher Simrock to create a version for viola and piano.
On October 14, 1894 Brahms wrote to violinist Joseph Joachim, asking whether they could meet in Frankfurt in the first half of Winter. Brahms mentioned that he would try to bring Mühlfeld and possibly a viola part for them to perform for Clara Schumann. Yet Brahms wrote again just 3 days later, stating “I hope that Mühlfeld can come – as I fear I find these two works quite awkward and unpleasant as viola sonatas.” This was before he had even created a version for viola!
There is evidence that Brahms sent Simrock a viola part in February or March of 1895, although no such manuscript is known to exist today. It appears that Brahms had little time or interest in this transcription. Indeed, as the publication date, June 1895 (the version for violin was published in July 1895), was approaching, he still had not completed the task. Brahms did, nonetheless—and perhaps out of friendship to Joachim—find time to produce versions for violin and piano which featured not only a complete violin part, but also a substantially revised piano part.
I couldn’t find any decent, succinct recordings, but Youtube has some of mediocre quality (mostly because they were recorded on a video camera, not because of weak playing).
Brahms Viola Sonatas, Op. 120