[srcreigh]

Why isn't it trivial to cut out that noise?

is it 4 motors? especially with data about current RPMs of the motors, with a FFT and some adjustments, it should be pretty trivial to deafen the sound.

From my listening many motors are effectively pretty simple instruments with normal harmonics etc which are easy to model. I could be wrong though.

[meatmanek]

There's going to be a significant white noise component due to the turbulent airflow, which won't be as easy to cut out.

[samatman]

There's so much white noise outdoors that removing the dominant harmonics that make drones, well, drone,† would make a huge difference in the palatability of being around them.

†The noise quadcopters make has nothing to do with why they're called drones, but is the perfect description nonetheless for the noise they emit

[williamtrask]

If they combined it with audio from the phone they would have a much better chance of isolating noise.