According to the linked article, scientists:
…recorded two different dunes: one near Tarfaya, a port town in southwestern Morocco, and one near Al-Askharah, a coastal town in southeastern Oman. No matter where recordings were made near the Moroccan dune, the sands sang consistently at about 105 hertz, in the neighborhood of G-sharp two octaves below middle C. The Omani sands also sang powerfully, but sometimes unleashed a cacophony of almost every possible frequency from 90 to 150 hertz, or about F-sharp to D, a range of nine notes.
So, why do the dunes sing?
His team’s hypothesis is that the vibrations of flowing sand grains synchronize, causing stretches of the sand grains to vibrate in unison. Their thousands of meager vibrations combine to push the air together, like the diaphragm of a loud speaker, Dagois-Bohy said. “But why do they synchronize with each other?” he noted. “That’s still not resolved.”
…the theory behind the sound still requires more elaboration to explain why, for example, the flowing sand still needs a thin layer of stationary sand underneath it to make a sound. He suggests the sliding sands resonate with similar-sized grains beneath the avalanche. Those buried grains may lie in chain-like patterns that intensify the resonance. “Once you have this resonance, the amplitude of the vibration will be large,” Patitsas said.
In other words, they’re still not sure. But the sound is very cool. Check it out.