Hi Bert, Check out this youtube video: http://www.youtube.com/watch?v=8V3OZMW_45M
These same Russian guys have several videos using different voltage powerlines. This one is 500kV and the deep sound must be 50 cycles. I think they are using some kind of little machine to launch a small wire across various powerlines and recording the resulting arcs.
This one is a loaded 500kV line, and the arc is obviously full load! You can hear the echoes for quite sometime. Any idea of what is going on here?