Possible Clues:

• Group of vocal people
• ''Messiah'' group
• Singing group often in robes
• Robed singing group
• Singers, choosy (not half!) I recognise at first
• Chapter on old Irish group of singers
• Musical group that practices at church
• Singers in church round one side of altar
• Endless task overwhelming current group of singers
• Organ manual which controls soft sweet-toned pipes