From Heavy Metal Makes Lighter Planes:
Qantas engineer Ian Salmon tested wing sections covered with a piezoelectric material that vibrates when a current is applied to it. When the tone of the sound was at its most effective pitch, Salmon’s wing panel achieved 22 percent more lift than it would have without the piezoelectric hum.
[...]
It’s all about changing the air flow from an unstable laminar flow to a turbulent flow that increases lift, Cummings said. The vibrations change the way the air behaves when it starts to break away from the wing’s surface, sucking it closer.