In 1926 Bax made the acquaintance of the Russian Countess and Count Benckendorff, who had settled in Ipswich. The Countess was the virtuoso harpist Maria Korchinska and her husband was a fine flautist. Bax drew inspiration from Korchinska's excellent technique. It was for her that he composed the Fantasy Sonata for viola and harp in April 1927. Korchinska's ability is clear in her two recordings of the piece. Korchinska, with Raymond Jerry, gave the first performance of the Fantasy Sonata in London's Wigmore Hall in October 30, 1927. In general, response to the first performance was positive, but, like all of Bax's chamber works, it has failed to find a place in the repertoire.

The egalitarian nature of the work is evident in Bax's switching between describing the work as "for viola and harp" and "for harp and viola." The harp does participate as an equal partner and exposes and develops much of the material. The first three movements proceed without pause in this relaxed work. Although there are no actual Irish folk song melodies in the piece, Bax's frequent use of modal melodies and dance rhythms gives it an Irish character.

Opening with a theme in the viola, the first movement, Allegro molto—Andante, is a sonata-form structure. The viola's melody, in the Dorian mode, provides the fundamental material from which other themes are drawn, and it reappears as a transitional passage between movements and one last time at the end of the last movement. Thus, the viola theme is the driving force behind the entire piece. The second movement, Allegro moderato, continues the level of energy of the first but presents new themes, both derived from the first-movement viola melody. A hearty, waltz-like melody gives the movement the feeling of a scherzo.

The transparent Lento espressivo provides welcome contrast in tempo and atmosphere, as does the full stop before the finale. Chords reminiscent of Debussy support a modal theme, played on the harp, that sounds almost improvised because of its loose construction. The secondary theme, however, is a soaring melody on the viola above harp arpeggios. Bax distributes the return of the first theme elegantly between viola and harp, emphasizing the equality of the two instruments. Marked Allegro moderato, the last movement renews the energy of the first and is filled with dance rhythms and melodic turns that evoke Irish folk music. Some have found in the scheme of the Fantasy Sonata a narrative in which a character—the opening melody for the viola—introduces more characters, the separate movements in between appearances of the theme.

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