[NotYourLawyer]

I was thinking that you’d need a pretty accurate clock reference to do this, automatically triggering based on a microphone to log the time.

But I guess not. Sound is slow on a distance scale of miles, so timing it to the nearest second is good enough.

[elteto]

A person's auditory reaction time is probably below 1s and that would throw off your calculation by ~340m. Most farms are larger than 340m in any direction so you still end up within the same property area.

[throwup238]

GPS sync allows phones to have accurate time to within microseconds if not hundreds of nanoseconds. Since sound travels ~300 m/s even 100ms only adds about 30 meters error. That's more than accurate enough to pin point a plot of land.