The Apple Hill Chamber Players are doing the first of five chamber recitals for Arts Across the River. Their program is two of the great chamber pieces, the Brahms Horn Trio and the Mozart E-flat Piano Quartet as well as the less well known Piston Duo for Viola and Cello.

Goerge Houle, an outstanding musician, is the new director of the New York Pro Musica. As one of the groups that "started it all," they are still most always worth a hearing. One of the numerous events in the B.U. Celebrity Series.

The Bach Society Orchestra begins its season with an all-warhorse program spanning the period 1723-1809. It should suit those of conservative taste. At $1, the Bach Soc is the proverbial "entertainment bargain." --K.T.H.

DUNSTER HOUSE LIBRARY. Mary Beekman, positiv organ, playing selections from the Bach Klavieruebung. Free. Friday, October 19, 5:30 p.m.

BURDEN HALL (B-SCHOOL). Apple Hill Chamber Players. Works of Mozart, Piston, and Brahms. Admission: $3 (students: $1.50). Friday, October 19, 8:00 p.m.

JORDAN HALL. New York Pro Musica. Tickets: 536-2412. Saturday, October 20, 8:30 p.m.

SANDERS THEATER. Bach Society Orchestra. Bach: Suite #3; Haydn: Symphony #101, and Beethoven: Piano Concerto #5 (Hugh Wolff,scott). Tickets: $1. Saturday, October 20, 8:30 p.m.

DUNSTER HOUSE LIBRARY. Alexander's Feast, consort. Medieval, Renaissance, and Early American music. Free. Sunday, October 21, 3:00 p.m.

SANDERS THEATER. U.S.S.R. Baroque Ensemble. Tickets: Free from Peabody-Mason Music Foundation, Box 153, Back Bay Annex, Boston 02117. Wednesday, October 24, 8:30 p.m.