"They are an unbearable insult to his pride because they tell front-line North Korean troops, most of them sons of the elites, what they are not supposed to hear," said Shim Jin-sup, a retired psychological warfare officer in the South Korean military. "They help undermine the total information blackout in the North, Kim's dignity and the very foundation of his regime."
For the South, they fall under the military's psychological warfare operations, a bloodless escalation to be deployed in the endless pattern of provoke-negotiate-repeat that has characterized North-South relations since the Korean War ended 60 years ago.
The South's Defense Ministry says that the broadcasts advertise the "superiority of a free democracy," the "happy life of South Koreans" and the "true reality of North Korea," as well as news about the outside world. The pop songs, which also included hits by the feathery-voiced chanteuse IU and the macho-ish boy-band Bigbang, were aimed at giving North Koreans get a taste of South Korean youth culture, the ministry said.
More from the New York Times:
Kim Jong-un says North's 'military muscle' made Korean deal possible
Deal between North and South Korea defies their mutual disdain
Koreas agree on deal to defuse tensions
"We do this so that North Koreans can learn that the world is changing," Kim Min-seok, a ministry spokesman, said in a briefing. "We believe this helps significantly improve human rights of the North."
Officials say that depending on the weather and other conditions, the broadcasts can travel up to 12 miles over the border, reaching mostly North Korean border guards but also some front-line villages.
Stung by the international news media's characterization of the broadcasts as propaganda, the Defense Ministry insisted that they be referred to instead as a "voice of hope."
"If propaganda attempts to change the other side through distorted and exaggerated facts, what we do is not propaganda," Kim Min-seok, the ministry spokesman, said.
These "voices of hope" include calling Kim Jong-un "childish" and "lacking confidence," according to military officials who heard the recording.
"Kim Jong-un's incompetent regime is trying to cheat the world with absurd lies," was another, according to a snippet captured by a South Korean cable channel.
"Soldiers of the Korean People's Army!" one recording said. "Don't waste your beautiful youth for the North Korean dictatorship but rise up against it!"
Read MoreNorth, South Korea agree to ease recent tensions: Yonhap
Not to be outdone, the North began broadcasting its own propaganda, but its decrepit sound system, handicapped by the country's spotty electricity, went largely unheard on the South Korean side, South Korean officials said.
Emblematic of the two countries' relative economic health, the fences on the South Korean side are ablaze with blinding lights at night to detect infiltrators, while the North is shrouded in darkness.
Dueling propaganda loudspeakers are hardly new. Germany and France exchanged loudspeaker taunts across their front lines in World War II.
Music joined the arsenal as early as the 1960s, when the East blasted Russian martial tunes over the Berlin Wall and the West answered variously with Dixieland and noise trucks.
The Koreas have harangued each other at the border since 1962, with North Korean loudspeakers urging southerners to defect to the "socialist paradise," a "country with no taxes," and on the south side a giant electric billboard flashing the number of cars in "Free South Korea."
Both sides deployed female voices, echoing over the hills day and night, to lure border soldiers to defect.
Even after the North's economy collapsed in the 1990s, the loudspeaker war raged, until 2004 when, during a period of improved relations, both sides agreed to desist.
The speakers remained off until Aug. 10, when the South flicked them back on in response to the wounding of two soldiers by land mines that Seoul said were planted by the North.
Read MoreN.Korea and S.Korea have first major border clash in 5 years
If Kim Jong-un feared that his troops, spellbound by the synthesized siren-call of K-pop girl groups, would mutiny, that did not happen. More likely, the broadcasts mainly prick his cult of personality.
In the North Korean textbooks, Mr. Kim's grandfather, Kim Il-sung, single-handedly liberated Korea from Japanese colonial rule. He could turn sand into rice and pine cones into grenades, according to North Korean books and animations. Mr. Kim's father, Kim Jong-il, held 1,200 titles, including "the Guardian of the Earth."
Mr. Kim, whose favorite title is "the supreme dignity," has executed generals for offenses as minor as dozing off in his presence. His uncle was executed for, among other crimes, "halfheartedly clapping his hands" when Mr. Kim entered a conference hall.
Loudspeakers from across the border represent one of the few public channels of information he cannot control. The United States-sponsored Radio Free Asia is another. Activists also regularly organize balloon drops, sending hydrogen balloons across the border carrying transistor radios, anti-government leaflets, and DVDs and computer memory sticks loaded with South Korean movies and soap operas.