If the problem is centred on industrialised areas of the world, that sugggests industry as the root cause. So the conspiratorial aspect of the question should really be the failure to hold industry to account.
To be fair to investigators, the vibrational nature of sound can make it very difficult to identify an exact point of origin. I have party walls with my neighbours on either side. I've discovered more than once that a working drill or other tool can be heard through both walls at the same time, and it's impossible to tell which neighbour is really being the handyman. (And whichever one is not the handyman is probably blaming me) When one of my neighbours is in his garden, his voice "bounces" off hedges in such a way that it seems to come from the other side.
To be fair to investigators, the vibrational nature of sound can make it very difficult to identify an exact point of origin. I have party walls with my neighbours on either side. I've discovered more than once that a working drill or other tool can be heard through both walls at the same time, and it's impossible to tell which neighbour is really being the handyman. (And whichever one is not the handyman is probably blaming me) When one of my neighbours is in his garden, his voice "bounces" off hedges in such a way that it seems to come from the other side.