What is causing the 'trumpets in the sky'?

This wave form shows how the sound was recorded when captured in Canada

By Rick Moran

This story made the hair stand up on the back of my head when I first read it. And when I heard the sounds coming from the sky - noises sounding like trumpets blaring - I 
became intensely curious.
Hundreds of recordings of these "trumpets in the sky" have been made by ordinary people all over the world, in both hemispheres. The sounds are sometimes incredibly loud. Scientists have offered a couple of rational explanations but so far, no theory has been proven.

Daily Mail:

A mysterious noise from the sky is continuing to baffle people all over the world - as well as giving those who hear it sleepless nights.
Sounding like a trumpet or a collective from a brass section of an orchestra, a selection of videos shot from the Canada to Ukraine, via the U.S., Germany and Belarus show strange goings on above us.
And the eerie sounds have been continuously heard at all different times and locations for almost a decade. 
The first video posted on YouTube recording the unusual, unearthly sounds, was in 2008 when a user recorded the strange sounds in the sky from Homel, in Belarus. 
That same year another anonymous user shared the 'ear-deafening' sounds that they insisted 'were not a hoax,' from a quiet neighbourhood believed to be in the U.S.
Kimberly Wookey from Terrace, British Columbia in Canada first captured the alien sound in June 2013, and since then she has managed to capture several recordings of the noise with her most recent being on May 7 this year. 
Writing on YouTube, where she posted the videos, Ms Wookey writes: 'On the morning of August 29, 2013 at approximately 7:30am I was awoken by these sounds. 
'I shot out of bed realizing it was the same sounds I had heard before and I ran looking for a camera to try to capture them with. I came out into the living room to find my seven-year-old son awake and scared wondering what was going on. He had said the noises woke him up as well and shook his window.
'I managed to record three clips showing almost five minutes of these strange sounds. After it was over and I sat down at the computer to upload the video. After checking my Facebook I noticed a lot of locals had heard the same sounds again but this time it was far more widespread. 
'I have no idea what these sounds are but it is pretty strange and I am glad that I was able to catch them this time and share what I heard. The sounds were heard again on Sept 8th at 6:30am so far we have confirmed reports of it being heard from town to the lake, 25km away.'
So what does the mother of four think the noises are?
'I personally do not believe this has any religious connection, nor do I believe it is aliens, graders, trains, construction, etc,' she added. 
'I do believe it could be a geophysical phenomenon.

I can't believe this phenomenon has been around for only a decade or so. The fact that we didn't discover how universal it was until people started to examine social media sites like YouTube suggests that what was once individual, far flung reports that seemed unrelated at the time turned into a world wide phenomenon once crowd sourcing became common.
One Russian scientist has an interesting theory:

In our opinion, the source of such powerful and immense manifestation of acoustic-gravity waves must be very large-scale energy processes. These processes include powerful solar flares and huge energy flows generated by them, rushing towards Earth's surface and destabilizing the magnetosphere, ionosphere and upper atmosphere. Thus, the effects of powerful solar flares: the impact of shock waves in the solar wind, streams of corpuscles and bursts of electromagnetic radiation are the main causes of generation of acoustic-gravitation waves following increased solar activity.

Given the surge in solar activity as manifested itself in the higher number and energy of solar flares since mid-2011, we can assume that there is a high probability of impact of the substantial increase in solar activity on the generation of the unusual humming coming from the sky. It should be pointed out that solar activity began to rise sharply since early 2011, with its amplitude significantly higher than all forecasts given by a number of influential scientific institutions in 2010 and 2011. Meanwhile, the observed increase in solar activity is fully consistent with the forecast of the International CommitteeGEOCHANGE published in the Committee's Report in June 2010. If this growth rate of solar activity continues, its amplitude by the end of 2012 will be higher than the amplitude of 23rd solar cycle, and in 2013-2014 the solar activity will reach its peak the amplitude of which was predicted by us to be 1.5 - 1.7 times higher than the amplitude of the 23rd cycle. 

What I like about this theory is that it accounts for an increase in the number of acoustic events in recent years. It could be that this phenomenon has been occurring with much lesser frequency in the past and occurring over uninhabited parts of the planet. Ergo, fewer people would hear it and fewer still would report it. 
Whatever it is, the acoustic events point up the fact that we still have a lot to learn about the atmosphere - except for global warming know it alls who have it all figured out. Maybe we should ask them what the sounds are? No doubt they will chalk it up to human industrial activity.

go  here  and  here  for videos and sound tracks from  around the world.