Related Clues:

• Treble clef singers
• Some glee clubbers
• Vocalists
• Male singers
• Male singers from Reston
• Lowest female singing voices
• Male voices
• Male singers with high register
• Male singing voices
• They sound high enough for getting on for a dozen alternatives, perhaps (6)
• What sound like notes hit the high notes
• Perhaps invoices with a slot
• Note how deep these girls get
• There's nothing in the last that makes them so low, by the sound of them
• High church hymns, by the sound of them