Historical clue formulations

• What's heard exactly twice in a lifetime? — 2025-09-24
• Nighttime sound? — 2025-08-07
• White noise? — 2020-11-05
• Repeated part of a five-mile hike? — 2017-04-16
• Hi-fi sound? — 2015-03-22
• Sound of silence? — 2012-11-03
• End of July by the sound? — 2011-09-04
• Second in line? — 2010-09-22
• iPod sound? — 2010-01-03
• Piece of pi? — 2008-11-07
• One of a finite pair? — 2003-05-01