Possible Clues:

• 'Whaaa?'
• 'I don't get it'
• Mellencamp: "Uh-___"
• Quiet without silence, initially — what?
• Befuddled comment
• 'I didn't catch that'
• “Say what?”
• Come again, though only occasionally
• 'You mean ... what?'
• ''Beg pardon?'' but way less formal