Possible Clues:

• Cheek to lower sound for this performance!
• Move the mouth as if saying or singing a recording
• Mouth opens, letting investigating periodontist see your nasty caries
• Sly pinch nearly ruffled mime
• Just pretend to sing
• Move mouth in time with insolence and unison
• Singer on TV might use such clips possibly around openings to youthful numbers
• Emulate Milli Vanilli
• Fake it, musically
• Mouth opens, letting in pea soup — your new cuisine