Can you simply control the volume by multiplication (or rather division) with a parameter that is based on the distance between drone and controller?
Or create a curve that goes from 1 to zero, where 1 is associated to “0 distance” and zero with “100 m distance”, for instance. Plug in the distance between drone and controller to find the value that corresponds to the respective distance. With the curve’s falloff you can control how much “quieter” (does that word exist???) the drone becomes when it flies away.