Hi, Bob, that's interesting, and not surprising.
I have very fond early memories of resting my
head on my mother's chest while being carried
in her arms - it was so fabulous to hear her
heartbeat and her joints/body creak and move.
It was even better when she walked - then I
got that lurching sway as well as the warmth
and sounds. It was a sad day when I got too
big to be carried like that!
I had the honor of being present with a girlfriend
during her labor last week. You often hear that the
fetal heart monitor is distracting during labor, but
we both agreed that in the pre-dawn hours, it was so
lovely and relaxing to listen to that rhythm.
I bet there are many 'new age' / relaxation CDs
available that take advantage of heart sounds to create
relaxing soundscapes. One of my favorites is Anugama's
CD Shamanic Dream - but that could be partly due to
having heard it for a year or so in the context of a
yoga class I was taking at the time. Whenever I hear
it, I fall immediately into a more relaxed state.
I think there are probably other sounds that call a
similar relaxation response - such as the sound of
waves on the beach, or water moving along stones.
One of my favorite CDs that combines water, heartbeats,
and natural sounds (and no instrumental or vocal music or
other such unnecessary distractions) is Dolphin Dreams
by an artist whose name I forget. It is uncannily transportive -
I used to use it when I taught yoga at the YWCA and needed a
sound buffer - sometimes, with the echoes in the room, I would
feel literally overwhelmingly 'at the beach'. ;) By the way,
the musician developed it for pregnant and birthing mothers -
specifically his own wife.