Possible Clues:

• Confess
• Audition for "The Voice"
• Perform a big hit
• Harmonize with others
• Warble or croon
• Perform a carol, e.g.
• Squeal – or make a whistling sound
• Turn informer, regularly using espionage
• Vocalise; confess
• Confess last of moral offences, becoming good