Gloomy Sunday

'Gloomy Sunday' is a mournful song about a young man deciding to follow the woman he loves into death, written and recorded in Hungary in 1933. A composer named Reszo Seress wrote it shortly after breaking up with a long-time girlfriend, and it quickly became popular, but also began to develop an odd reputation as the 'Hungarian Suicide Song'. A number of deaths were linked to Gloomy Sunday, with suicides following a

playing of the song, or lyrics from it appearing in suicide notes. Shortly after he approached her seeking a reconciliation, the composer's girlfriend killed herself, leaving a note reading only "Szomorú Vasárnap" -- 'Gloomy Sunday'.

An English language version was recorded, and Gloomy Sunday's reputation as the suicide song spread across Europe and America, to a lesser degree than in Hungary, but still worrying authorities to the point of banning it from radio in several places.
(This is all true. There are websites that detail the song's history or debunk the 'suicide song' legend, and various recordings available for download.)
Now, a dusty and rather scratchy original recording of the song being played by its composer has been restored and duplicated with a clarity never heard before, and the original Hungarian version of Gloomy Sunday is regaining popularity -- but this version of the song seems to have a power over its listeners that none of the previously mass-marketed recordings came close to matching. Listeners find themselves babbling snatches of Hungarian they don't understand, experiencing terrible and sourceless fears, seeing strange visions they can't explain, and killing themselves at a frightening rate.

Possibilities

1     Seress was a cultist, and quite mad. Along with the strange and troubling music he composed, there were rituals venerating forgotten gods. He forced his fiancee to join him, and at first she went along with it willingly, humouring a man she loved and believed could be brought back to sanity with patience and quiet surroundings. But soon the forbidden knowledge she was learning through their descent into the mythos began to horrify her. When she protested, Seress threatened to kill her if she backed out or if she told anyone of what they were doing.
His efforts were building towards a final rite he was reluctant to tell her about, but her own researches gave her an idea of what he was planning, and the sick fear of it became too much for her: she killed herself. Seress was furious at her betrayal, but continued his preparations for the rite as though he believed there would still be a second participant.
Now, the specially-prepared Gloomy Sunday recording acts as a kind of trap for the woman's spirit, periodically forcing her to inhabit and possess a listener's body against her will. It also calls to Seress, who immediately gives chase, armed with magical methods to force her to join him in the final ritual. The repeated suicides are the unfortunate ghost's only way to free herself from the trap.
If nobody intervenes, the deaths continue until Seress finally captures his late fiancee and goes ahead with the ritual.

2     Seress wasn't much of a composer, but he was a charming man with a wide range of acquaintances, and an accomplished plagiarist. One of his friends was a German-Hungarian scientist experimenting with brainwave transference using salvaged mi-go technology and money provided by Hitler's Reichstag. He was exploring a promising method of transferring thoughts and memories from one mind to another encoded in repeating tones like the melody of a song. Seress liked the 'music' he heard in the scientist's home one night, and left with the recording hidden under his coat, eventually publishing an adapted version of it with lyrics written by another acquaintance, as Gloomy Sunday.
Now listeners are experiencing some of the scientist's recorded thoughts as they listen to his brainwave-duplicating sounds, including his growing doubt and guilt over accepting money from nazis, and increasingly paranoid fear of what can only be mi-go attempts at intimidation in order to reclaim their equipment.
If nobody intervenes, the Gloomy Sunday reputation continues to grow and the recording's publisher quietly takes that version off the market. Isolated suicides continue, as private copies still exist.
3     Gloomy Sunday is simply a sad, regretful song with nothing supernatural about it. The wax the recording is pressed from is contaminated with toxic chemicals that fill the air with poisonous fumes when played. The hallucinations and deep depression listeners experience are symptoms of exposure.
In time, the recording company will switch to a safer material and try to hush up the deaths, leaving thousands of potentially deadly records in the hands of the public.