This is a theory that I just formed. Or at least it clarified a bit.

It sounds like perhaps something like one person in 1000 can clearly hear "Ear Tones." And the information that I have collected from different people indicates to me that two people will usually not hear them at the same time. My latest theory proposes that each person might be sensitive to electromagnetic energy field waves or radio waves which have different frequencies.

It could be that before a given earthquake only signals having certain frequencies are being generated. Or it could be that quite a few signals with different frequencies are being generated at different times. And because of those frequency differences each individual may hear ones which are generated at specific times. Those times will not match the times when other people hear them.