Hackers ripped off an estimated 50,000 music files, involving Michael Jackson's entire back catalog, from Sony's internal music-sharing site.
The attack was discovered nearly a year ago during routine monitoring of social networking sites, the Sunday Times reported.
Sony Music was in possession of the files after striking a March 2010 deal worth at least $200 million with Jackson's family, according to reports.
Sony inherited unreleased material from some of the King of Pop's most notable albums, including “Thriller” and “Off the Wall."
While Sony has not confirmed the amount of music that was stolen or the artists whose work was involved in the attack, two suspects were arrested not long after the attack was detected, reports said.
James Marks, 26, and James McCormick, 25, pleaded innocent Friday at Leicester Crown Court in central England, according to reports. The pair was charged with computer misuse and copyright offenses.
The breach apparently occurred on the heels of a much bigger intrusion that took place in April 2011, when hackers compromised the Sony PlayStation Network to steal the personal information of tens of millions of gamers.
Sony experienced multiple other breaches following that attack.