There's also natural Earth sounds down at 5 - 20,000 Hertz. Have to be away from powerlines to hear them though. I built some gear to see what I can hear, but being close to the equator and powerlines I don't hear much other than sferics and tweeks from lightning.
Search for the recordings of Steve McGreevy.
http://www.youtube.com/watch?v=eHvdZ...eature=related