Possible Clues:

• Vowel in the word 'rude'
• 'Crude' vowel sound
• 'Rude' sound
• 'Cute' vowel sound
• 'Cute' sound
• Sound in "cube" but not "cub"
• Sound from flutes and tubas
• Sound of music?
• Extended vowel sound, as in 'cute'
• Mule sound