Possible Clues:

• With 31-Down, lilting song syllables
• Singer's nonsense syllable
• Carefree singing syllable
• 'The flowers that bloom in the spring, ___ la'
• Lead-up to la
• 'La la' preceder
• Butterfly, once
• "___ la la . . ."
• La-la intro
• Syllable in some refrains