Possible Clues:

• Male singing voices
• Choir voices
• OR x 10 is enough to make the men sound high
• High church hymns, by the sound of them
• Notes sung by the 10?
• They sound high enough for getting on for a dozen alternatives, perhaps (6)
• Some Met stars
• What sound like notes hit the high notes
• Male singers from Reston
• Pavarotti and the like