Hmmm.

> the listening circuit must also be tuned to resonate at the expected frequency of proton precession, which will depend on Earth’s magnetic field at your location

> the frequency of these tones matches the magnetic field at my location to about 1 percent

I don’t doubt the physics, but I’m not sure about the experiment design. Being able to hear the correct frequency may just mean you’ve built an oscillator and tuned it.