BILLED BY ITS promoters as a symbol of friendship between the Vietnamese and American people, Sunday's concert at Sanders attracted a small but enthusiastic audience. Violinist Joseph Silverstein, concertmaster of the BSO, and pianist Anton Kuerti, artist in residence at the University of Toronto, presented a varied and exciting joint recital to raise money for the rebuilding of Bach Mai hospital, which American bombers destroyed a little over a year ago. The musicians gave no speeches; their playing was eloquent and moving.

Kuerti presented the Liszt B minor piano sonata, a huge and difficult piece. Its one overblown sonata-form movement takes almost half an hour to perform. In it, Liszt explores extremes of pitch, timbre and volume as he takes two themes through striking and dramatic transformations.

In its time (1853) this sonata was a radical work. Its innovative harmonies and grandiose scope threatened and bewildered earlier Romantics who concentrated on shorter lyrical forms. However, Wagner, in a letter to Liszt, wrote, "The sonata is beautiful beyond all expression, great, gracious, profound, noble, sublime like yourself."

Kuerti's performance displayed the necessary grand contrasts and driving rhythmic energy. Given the unrelenting technical demands of the work, the many notes he missed did not dispell a highly virtuosic effect. The performance was dramatically unified and entirely convincing.

Silverstein played Bach's Second Partita for solo violin, one of the rare pieces for unaccompanied violin. The difficulty of rendering harmonies and melodies simultaneously on the violin's four strings scares most composers away. Bach surmounted this limitation with an ingenious mixture of broken chords and double stops. He could thus create the illusion of a many-voiced instrument--he even wrote a convincing three-part fugue for solo violin.

This partita demands considerable sustained energy; the violin is never at rest. Long passages of uniform rhythmic figures challenge the performer's musicianship.

Silverstein responded to these demands with self-assured poise. Almost motionless on stage, he produced a rich and seamless tone throughout the piece. Like the voice of a singer who never has to breathe, the sound never faltered. His phrasing was somewhat unelastic during the first movements, but the final Chacconne, a great test of violin virtuousity and musicianship, was a model of Bach interpretation, expressive yet tightly disciplined.

In addition to these solo works, Kuerti and Silverstein collaborated twice. The Schubert B minor rondo with which they opened the program was disappointing. The performance was disjointed, the phrasing stiff, the lack of rehearsal evident. Insensitive and brusque playing submerged Schubert's lyricism.

The A major sonata of Cesar Franck which concluded the program fared much better. It is a pleasant if somewhat syrupy work that continues the overt emotionalism of Liszt without any of the daring and grandeur that could make it any more than mediocre.

However, Kuerti and Silverstein played it beautifully, extracting all the possible drama and depth from this lollipop. Their playing was relaxed and expressive; Silverstein even added a number of romantic-type slides to his playing, and ended the concert with excitement and style.

This was a benefit concert, but one devoid of exhortations or affirmations of moral fervor. More eloquently than any words, the music bespoke the humanitarian impulse which had brought these musicians and listeners together.