It's radiation pressure from the pulsed beam of light moving the tiny microphone. MEMS microphones are tiny capacitors that are vibrated by sounds. In section

It's radiation pressure from the pulsed beam of light moving the tiny microphone. MEMS microphones are tiny capacitors that are vibrated by sounds. In section IV.C of the paper, they test whether the effect is mechanical or photoelectric, and determine that it's acting via mechanical vibrations, since the effect is stopped by gluing the microphone down with a transparent bit of glue. Think of it as a tiny solar sail-- they're hitting a very small piece of metal with a lot of photons, so even minor deflections are translated effectively. "The diaphragm is a thin membrane that flexes in response to an acoustic wave. The diaphragm and a fixed back plate work as a parallel-plate capacitor, whose capacitance changes as a consequence of the diaphragm’s mechanical deformations as it responds to alternating sound pressures. Finally, the ASIC die converts the capacitive change to a voltage signal on the output of the microphone." "As can be seen, the modification decreases the amplitude of the signal detected by the microphone, and the signal after the glue application is less than 10% of the original signal. We thus attribute our light-based signal injection results to mechanical movements of the microphone’s diaphragm, which are in turn translated to output voltage by the microphone’s internal circuitry."