Although it seems that there are some people just playing recording of the actual sounds, I believe that this video may give an answer for the legit
recordings.
feel free to disagree, this is not a pet project or belief

just thought I would share what i found
This a BBC documentary from 2008, long before any sounds were heard
Notice at the .45 min mark how the sound is much like what has been heard and then at the 1:23 mark he gets into the meat of it and finally at the
2:30 he say's that impending volcano eruptions or blocked volcano's will emit sounds.
www.youtube.com...#!
and the below may give an answer for why people say that these sounds seems to be coming form the sky, the sky is being used as a sound board.
The skyquakes are caused by harmonics deep inside the Earth traveling from a mutating core through the layers of the planet to the surface. The sky
acts as a sounding board capturing and magnifying the sonic and electrical frequencies shot from the center of the world.
The stresses building up in the core deep in the heart of the world also migrate outwards to the mantel and eventually the crust. Slipping strata
within the bowels of the planet create incredible pressures…pressures so great that temperatures soar and dense metals flow like liquids. [See
Doomsday: Earth's core spinning out of control]
Water becomes a superheated plasma and interacts with layers of quartz crystals emitting harmonics audible as a hum and also generating earth lights
and skyquakes echoing across the sky
beforeitsnews.com...:_Warnings_From_Earths_Destabilizing_Core.html