Bong! a B-flat punctuates the silence.

"Yo, yo, yo, yo, yo, yo, yo, yo, yo," the voices bound up and down the scale.

Bong! resounds the piano.

"Ho, ho, ho, ho, ho, ho, ho, ho, ho," the perfectly pitched voices now reach lower and lower.

And then, after a few more vocal calisthenics, members of the Foundry United Methodist "Cathedral Choir" turn to the rest of their bodies. Stretching, bending and twisting, they coax tense muscles and tight diaphragms into shape.

"Time for back rubs!" announces Eileen Morris Guenther, their young and energetic organist and choir director. In train-like formations, they massage away each other's after-work tensions.

"Okay, now karate chop! Up the left side . . . down the right side," Guenther continues to their less than melodic tune of sighs.

And so begins the midweek rehearsal.

At 11 a.m. Sunday it all comes together as the 44 singers, in red cassocks and white surplices, ascend to their places of honor on either side of the altar. Carrying music carefully annotated to indicate places to breathe or tricky Latin or German pronunciations, they left their voices with the organ, and once again the handsome church on 16th Street is filled with a symphony of sound.

Afterwards they celebrate over brunch.

Whether they warble complicated Bach cantatas or garble traditional anthems, church choirs are among the most dedicated and cohesive of church groups.

"They're committed 10 months a year, twice a week," with time off only during the summer, said Donald Sutherland, music director at Bradley Hills Presbyterian Church. "Outside Sunday school teachers, I can't think of any church group that requires that kind of commitment."

Not only do they sing and rehearse together an average of five hours a week, they socialize, job and apartment hunt for one another, and sometimes marry each other. They reschedule business trips, pare down working hours, juggle social engagements and sometimes commute great distances to make rehearsals and performances. They serve as sorrogate families to singles away from home.

Besides offering a spiritual outlet to the religious minded, church choirs can be a prestigious forum for Washington's abundance of highly trained musicians. When church hunting, trained singers often look for an excellent choir and view a church's denominational ties as secondary.

And, as many choir directors proudly point out, their ranks are swelling even at a time when overall membership at their churches is waning.

Choirs can be a way of life.

Carole Hilty, for example, says, "Life without church music would not be worth living," and so when she moved here two years ago, "one of the first things I did was search out a church with a good choir. I heard the Foundry choir perform the 'Messiah' and I was impressed."

A month later, Hilty, 39, who has studied music and voice most of her life, was performing with the choir. She now counts her best friends from among fellow choir members at Foundry.

David Curfman's love of church music was so intense that throughout his 10 years of medical school and residency program he spent many of his rare free hours directing a choir and performing as an associate organist at Grace Lutheran Church on 16th Street NW.

"When you think of physicians you think of golf," said Curfman, 29, a neurosurgeon. "Well, in a way that was my golf game." When Curfman opened his own practice several years ago and time demands grew even greater, he had to restrict performances to special occasions. But he still lectures on church liturgy and music.

His love for the music is mirrored by 40 mostly downtown professionals who spend their lunch hours every Tuesday at the United Church, where they sing with the Little Hamburg Chorale. Once a month at lunchtime the choir performs for crowds of up to 150 downtown workers.

And then there is Ken Yellis, who joined the Foundry choir 2 1/2 years ago as a means of supplementing his voice lessons and got hooked. "I enjoyed being part of something that was excellent," said Yellis, 37, "but I didn't know how important the choir was to me until my mother died last summer. I got a sense of overwhelming love and support from them, and that meant a lot to me."

In December, Yellis, who was born Jewish and raised in the Christian Science faith, was baptized and joined the church. Such conversions and church jumping are not unusual, say choir directors.

Beyond the spiritual and artistic fulfillment and fun, participation in a choir means hard work.

"My choir is sweating when they leave rehearsal," said Forrest Henderson, a new choir director at Asbury United Methodist Church in Northwest Washington. Henderson, like Guenther, leads warm-up exercises complete with back massages before rehearsing. "They're not used to that kind of workout," but it has paid off, said Henderson, who reported that since he took over the choir, the congregation has broken out into applause during workship services.

"When you've worked hard and had a particularly vigorous rehearsal you sometimes come out feeling absolutely washed out and exhausted," said Hilty, who has directed church choirs of her own. "One Sunday after singing a Bach cantata I came out dripping. There wasn't a legitimate place for a soprano to breathe!"

But it is more than the singers who work.

"We pore over hundreds and hundreds of pieces of music to find a few anthems each week," said Bradley Hills' Sutherland.

Foundry's the Rev. Edward Bauman, nationally renowned for his preaching, gives sermon texts months in advance to Guenther who then "spends a fantastic amount of time choosing music" to complement the theme of the liturgy.

"Music has always been a high priority at Foundry," said Bauman. "We see it as part of our ministry. It brings people to church, there's no doubt about it. I have people tell me they come just to hear the music."

Good music is expensive. Sheet music, upkeep of organs, pianos, robes and paid soloists and musicians cost money, which many clergymen would rather funnel into repairing a leaky roof or hiring additional staff.

Foundry's annual music budget (including staff salaries) is about $32,000, while the Shrine of the Immaculate Conception, for example, spends $100,000 a year on its extensive music program and New York Avenue Presbyterian Church allots $25,000 for its 80-member volunteer choir.

To ensure the highest quality musicianship, some church choirs will not accept volunteers and will work only with paid professional singers. Many of the widely acclaimed local church choirs like those at the Shrine, Washington Cathedral, St. John's Episcopal Church at Lafayette Square and the Episcopal Church of the Epiphany pay their members up to $300 a month for their services.

Other church choirs like those at St. Matthews Catholic Cathedral, Georgetown Presbyterian, National City Christian and National Presbyterian are partly professional.

Foundry, which has a fast-growing reputation among leading area choir directors, has a paid quartet of soloists who "add a little sheen" to the choir, said Guenther, 33, who holds a doctorate in organ.

"My voice lessons are just too expensive to give it away," said Alburtt Rhodes, Foundry's paid tenor. Rhodes, who is Roman Catholic, got his start in Catholic churches but takes on "whatever religion I happen to be singing for." Currently Rhodes earns $14,000 a year performing but hopes soon to leave his position as a legal secretary and pursue a full-time career in opera.

Although Guenther, Sutherland and many other choir directors welcome the "sheen" that professionals such as Rhodes can add, they say community spirit must come first, and so they are reluctant to make membership in their groups exclusive.

"Sometimes you have a shudder at the voices you have to work with," said Sutherland. "But in a church atmosphere you can't turn people away."

Only once has Guenther turned someone away "when it just wasn't going to work. Usually everyone who wants to sing can match pitch."

Steve Prussing of New York Avenue Presbyterian Church solves the problem by giving free voice lessons to those who can't keep up.

The major problem church choir and music directors say they face now is inflation and what it might do to church budgets.

"Music is always the first to go when budget cuts come along," said Guenther. "Unfortunately it's not always viewed as a necessary part of religion."

"Any minister realizes a choir can really set up a church liturgy," said Audree Bauman, who, by the way, is married to Foundry's pastor, whom she met not so coincidentally while a member of the church's choir in the 1940s.

After watching his future wife perform a solo one Sunday, Bauman, who attended Foundry while stationed here during World War II, immediately joined the choir. "The story goes," said Mrs. Bauman, "that the first thing he asked the choir director [was] if the alto soloist was spoken for."