HYATTSVILLE, Md. — Rehearsal assistant David Mathers brought his lapel microphone up to his mouth for a throaty demonstration of the German word "dich," putting particular emphasis on the guttural "kh" sound at the end.

"We really want to hear that," Mathers said toward the end of a 2 1/2-hour choir practice. "The Holy Father's from Bavaria."

The comment caused chuckles — the singers know the pope is German.

But getting all the details just right for Pope Benedict XVI — down to every last syllable of a hymn in his native tongue — is serious business for this group of Washington-area Catholics. The 250-person choir will be singing at the papal Mass this month at Nationals Park.

Chosen from more than 500 people who auditioned, the group gathered for the first time March 10 at St. Mark the Evangelist Church in Hyattsville, just outside Washington. A week earlier they had received the 134 pages of sheet music, mostly for songs they had never heard before.

"What a gift to be able to see the pope," said Steve Dameron, a 51-year-old tenor from Morningside, "even if it's from the back."

Of course, this group won't just see the pope, they'll be his musical accompaniment. Their singing will be part of the experience for the 46,000 people who will pack into the new baseball stadium. Many more will watch the Mass on TV.

Tom Stehle, pastoral associate for liturgy and music at Our Lady of Mercy parish in Potomac, is serving as music director for the papal Mass. He worked for months to choose just the right music for the elaborate event, which will also feature performances by opera stars Placido Domingo and Denyce Graves. Along with familiar works like Mozart's "Ave Verum Corpus," the program includes international folk songs, gospel and music by contemporary composers.

In addition to the main choir, Stehle is coordinating a 65-person "intercultural choir," which will sing in French, Spanish, Zulu and other languages; a 175-voice children's choir; and an 80-person gospel choir.

"I wanted to use all of the creativity and imagination I could and also make it a very prayerful experience for everyone," Stehle said. "At the root, it's the prayer. It's not the spectacle, it's not the historic event. ... It's really about prayer, and what Catholics do every Sunday essentially."

This Mass presents some unique challenges. For instance, Stehle was concerned that choir members would not be able to hear the full sound of the group in the stadium, where there are no walls or a ceiling for the music to reflect off. To compensate, he devised an unusual seating arrangement.

In most choirs, all the sopranos sit together as a section, and likewise for the altos, tenors and basses. In Stehle's arrangement, smaller groups of sopranos are interspersed with smaller groups of altos, tenors and basses.

The hope is that the arrangement will help each singer hear the whole choir. That arrangement has its own challenges, as singers must be able to carry their parts without relying on those around them. The test won't come until the day before the April 17 Mass, when the choir will have its only opportunity to rehearse in the stadium.

The work is a labor of love for the singers, culled from parish choirs and other music groups throughout the Archdiocese of Washington, which includes the District of Columbia and five Maryland counties.

For his part, Stehle said he hopes the music won't just be beautiful, but will inspire people to join in. "Unlike anything that might happen in that stadium ever again, you might have 46,000 people singing their hearts out, which would be an amazing thing. That doesn't happen during the 'Star-Spangled Banner,' you know," Stehle said. "So that's what I'm looking forward to, to hearing the whole stadium just erupt, and have the pope go: 'Wow. This is the American church. This is a beautiful thing.' "