I found a good explanation of this "conspiracy" by Alan Cross at Global News Canada, titled The great 440 Hz conspiracy, and why all of our music is wrong:
...There is allegedly something sinister and evil about 440 Hz. It is said that the Rockefeller Foundation had an interest in making sure the United States adopted the 440 Hz standard in 1935 as part of a “war on consciousness” leading to “musical cult control.”
...this theory says that tuning all music to 440 Hz turns it into a military weapon.
...“The monopolization of the music industry features this imposed frequency that is ‘herding’ populations into greater aggression, psychosocial agitation, and emotional distress predisposing people to physical illnesses and financial impositions profiting the agents, agencies, and companies engaged in the monopoly.”
...It is said that propaganda minister Joseph Goebbels insisted that on 440 Hz tuning in Germany because he believed it made people think and feel in specific ways, making them “a prisoner of a certain consciousness.”
...“The powers that be are successfully lowering the vibrations of not only the young generation but the rest of us as well. These destructive frequencies entrain the thoughts towards disruption, disharmony, and disunity. Additionally, they also stimulate the controlling organ of the body — the brain — into disharmonious resonance, which ultimately creates disease and war.”