this is pure guessing but since the dead are supose to whisper would they have made them low frequency so the reatives could hear the dead speak more clearly? just a theorey nothing to back it up.

A very interesting theory. I would point to the modern analogy of using ELF/ULF frequencies that appear to have similar mood altering/neural inductive affects.

There are obviously those that dispute that ELF/ULF have any kind of affects on the human physiology, how can you explain that the CIA spent a decade and hundreds of millions of dollars on MKUltra?

Although I cannot quote a specific source, I do remember reading that extremely low frequency sound can in fact have similar mood/mind altering affects on people, and it appears that these structures have that very affect.

I'm aware that subliminal tapes use a descending tone scale to subconsciously move a person into a more receptive (Alpha?) state.

I saw a program about Stonehendge that theorised that one overlooked, or forgotten, aspect of that monument was to produce a sound resonance experience.