We have analyzed records of these sounds and found that most of their spectrum lies within the infrasound range, i.e. is not audible to humans. What people hear is only a small fraction of the actual power of these sounds. They are low-frequency acoustic emissions in the range between 20 and 100 Hz modulated by ultra-low infrasonic waves from 0.1 to 15 Hz. In geophysics, they are called acoustic-gravity waves; they are formed in the upper atmosphere, at the atmosphere-ionosphere boundary in particular. There can be quite a lot of causes why those waves are generated: earthquakes, volcanic eruptions, hurricanes, storms, tsunamis, etc. However, the scale of the observed humming sound in terms of both the area covered and its power far exceeds those that can be generated by the above-mentioned phenomena.
Otherwise, the strange sounds may be some form of control.
John Goodman explains the use of old Firehouse Sirens to freak people out.
Red State Official Trailer (language warning)
"Strange Sounds" DEBUNKED and EXPLAINED
"Strange Sounds" around the world explained. (HAARP? Project Bluebeam? Pole Shift? WRONG!)
Though nobody knows for sure still, some are convinced because of the amount of sounds in the large number of places around the world. Is it coincidence, or perhaps we have never noticed these sounds before? They may be electromagnetic, or techtonic, or otherwise. Perhaps a call like the noise the aliens make in War of the Worlds, or perhaps its just a call for us to look up in curiosity. Is someone trying to get our attention? Or someone just playing tricks?
http://www.youtube.com/watch?v=VgGFThlEeGE&feature=related I dont think its a hoax too many reporting these sounds...