That puzzles me, too. The thermal effect that drives a radiometer should be too slow to transmit audio. Radiation pressure transfer from photons is very weak, but wouldn't have bandwidth problems. The frequency response of this effect does fall off with frequency, but not very fast; see fig. 6. in the paper. There's still reasonable output at 3KHz.
Light pressure of sunlight is about 1mg/m^2. Or one nanogram per square millimeter, a reasonable size for a chip microphone. Will chip microphones respond to a nanogram of pressure?