The piece is titled Impromptu Op. 33 (Shostakovich later assigned the number to another work). It was found among the papers of violist Vadim Borisovsky of the Beethoven Quartet. It is believed it was written for violist Alexander Ryvkin of the Glazunov Quartet. The duet was written, apparently in one sitting, in 1931.
We do not yet know what this newly-found work sounds like. Shostakovich wrote one other work for the viola, the Viola Sonata (his last composition), which was written in 1975. You can listen to it here.