One nice thing about the Boston Symphony's open rehearsals is their concert hall quality. Instead of a tortuous practice session, a student audience sees and hears a preview of the next day's subscription concert. Charles Munch seldom finds it necessary to ask the orchestra to replay unsatisfactory passages. After nearly a week of practice, he has only a few subtle problems of interpretation to work out.

Still, Munch is a perfectionist. The string session had to play a passage from Roussel's Ballet Suite Le Festin de L'Araignee four times before it attained the precise dynamic contrast Munch wanted. Otherwise, the orchestra got through the rather dull score without mishap.

The rehearsal had begun with Bach's Suite No. 2 for Flute and Strings--chosen, no doubt, to show off the new female flutist, Doriot Anthony. Her crystal clear tone and prodigious breath control clicited applause from both audience and orchestra.

Beethoven's Fifth Piano Concerto was both the highlight and the lowlight of the evening. The orchestra was vigorous and forceful; Munch conducted with sweeping brilliance. But pianist Leila Goussean was as miscast playing the Emperor as Pier Angrli would be, playing Moby Dick. It takes a man--a strong man--to make this showy, difficult concerto come to life.