Possible Clues:

• Macho Irishman suppressing a source of harmony?
• Group of people that share an organ?
• Madrigal group
• Church group
• Group that works in harmony
• Group singing 'Hallelujah!'
• Invoices for a couple of dozen sheets, by the sound of it
• Preaching to the ___
• "You're preaching to the __!"
• Coldplay might hear a Roman Catholic one