Yogurt is quietly stirred. Foil-wrapped sandwiches are carefully uncovered. It is Thursday noon in the Folger Theater and another Midday Muse program is about to begin.

Now in its fourth year, the Folger Library's lunchtime series of concerts and poetry readings attracts each week 75 to 100 people who like a little entertainment with their lunch.

Yesterday the audience munched to Monteverdi with the Winter Night Quintet, which presented a program of madrigals.

Like the year-old Winter Night Quintet, Midday Muse performers are local people in the process of establishing their reputations. Reflecting their solid musical backgrounds, the five singers of the quintet brought considerable polish and even greater enthusiasm to a delightfully balanced selectionof madrigals. Some occasional problems, such as intonation difficulties and disagreements about the amount of vibrato to use, could be solved with time.

When the concert was over a teacher from the University of Rome pulled out a banana from her lunch bag. The concert had been a break in her research project at the Library of Congress. "It's a great thing," she said. "We don't have anything like it in Italy. But then," she added, dropping her banana peel in her sack, and perhaps reflecting on her country's more relaxed dining habits, "lunchtime in Italy is quite different."