Edouard Vuillard painted "Le concert matinal" during 1937 and 1938. It is connected with the Place Vintimille, which is now called the Place Adolphe-Max (as of 1940) in the 9th arrondisement in Paris. I imagine that Vuillard's musician friends came into his studio (notice that the music stands are artist's easels), and I imagine that the piece they might be playing is the Jongen Trio, Opus 30, because it is a piece of the period (1907), and also one of the few pieces written for violin, viola, and piano. The music on the stands also looks vaguely like the printed music. Here's the link to it in the IMSLP.
I wonder if the violist is the dedicatee of the piece? I wonder who he is? I wonder who the fiddle player is?
UPDATE: The picture is of two violinists: Leon Nauwinck and Madame Ortmans-Bach, and the pianist is Claude Crussard. The music is probably a baroque triosonata.
Le concert matinal, place Vintimille depicts an intimate scene wherein three musicians, two violinists and a pianist, have staged an impromptu concert. The "salon" in which the musicians have gathered is in fact Mme Vuillard's private chamber in the apartment she shared with her son on La place Vintimille. "The renowned violinist Léon Nauwinck was fond of painting; he knew that Vuillard enjoyed music, and consequently got into the habit of dropping by from time to time to soothe his morning doldrums, shortly before lunch, accompanied by Mme Ortmans-Bach and the pianist Claude Crussard. The easel and the drawing-table served as music stands. There were never more than three or four of us to enjoy these sessions, which were held in Mme Vuillard's bedroom, thereby adding fervour to the pleasure of listening to the music of Bach, Telemann and Vivaldi. Annette at Villerville and Bonnard's screen [both] figure in this masterly brush-and-distemper- sketch that us the very incarnation of life" (J. Salomon, op. cit., Paris, 1968, p. 177).