Possible Clues:

• Simon and Garfunkel number, say
• Two-singer song
• Song such as "Islands in the Stream"
• Song like "Take Me or Leave Me"
• Collaborative music
• "Telephone" or "When You Believe"
• The Righteous Brothers' "Ebb Tide" wasn't one
• Aria, occasionally
• Piece for brace fitting top of tooth
• "Ain't No Mountain High Enough" is one