Music has been used surprisingly often as an instrument of siege warfare:
During the US military's invasion of Panama to oust Manuel Noriega, psychological warfare specialists blasted rock music at the building he was holed up in, which ran counter to Noriega's personal taste for opera. One of the songs played was The Clash's "Should I Stay Or Should I Go?", the irony being the band opposed U.S. military interventions in Central America. Noriega eventually surrendered.
During the 1997 Tupac Amaru hostage-taking at the Japanese Embassy in Peru, police blasted martial music and opera around the clock to hide the sound of their teams tunneling into the embassy.
During both Gulf Wars, American tank crews would blast heavy metal with their loudspeakers, generally causing the Iraqis to run away or surrender.
At the siege of Waco, Texas, the National Guard blasted the holed-up Davidians with Barry Manilow songs.
And David Koresh and his Mighty Men (that was the name of their band - it's from 2 Samuel) responded with loud Christian rock Up to Eleven, until the power was cut off.
Guantanamo Bay, which uses rock and Barney songs as part of their "enhanced interrogation techniques".
At the battle of Gaixia (202BCE), Han forces outnumbered and surrounded Chu forces, but still couldn't break their defensive perimeter. Han forces began singing homesickness-inducing Chu folk songs, and the Chu army fell apart via desertion. The Han went on to rule China for over 400 years.
After the Battle of the Bulge, Nazis set up loudspeakers to pump out propaganda. One US corporal, who was Jewish, took it upon himself to set up his own loudspeakers to play the music of Jewish singer Al Jolson. That corporal was Melvin Kaminsky, who would later leave the Army to become an actor/producer/director of many films.
The Power Of Rock—or rather, the power of rock fans—was enough to win Finland the 2006 Eurovision contest. Traditionally the voting is mostly done around political boundaries regardless of actual music quality, but the year Finnish rock/metal band Lordi entered, Finland not only won their first victory but got a record 292 points.
Even more awesome because the Genre Savvy band knew how the system worked and entered the contest primarily to increase their name recognition in Finland- now they have fans world-wide.
In 1974, Portugal's entry for that year's Eurovision was broadcast at a certain time of day to signal the start of the Carnation Revolution, which successfully overthrew the country's 50-year fascist dictatorship.
A lighter use of Power of Rock involved Tina Turner songs being used as bird distress signals in Gloucestershire airport.
According to MythBusters, plants in fact do grow better when exposed to classical music... but grow even better when exposed to death metal!
Not quite rock, per se, but the Red Army during the Battle of Stalingrad used a variant of this trope: as they closed the noose around the Germans, loud speaker phones were set up to play tango of all things, mainly because it was felt they sounded sinister enough to demoralize the Germans. They would periodically interrupt with demands for the enemy to surrender, and then went right back to playing more tango tunes.
According to Andras Simonyi, Hungarian ambassador to the United States and former dissident leader, the music of The Beatles, Jimi Hendrix, and Cream, helped give Eastern European youth the resolve to stand up to their oppressive governments. Said Simonyi, "By keeping in touch with the music scene in the West, it kind of kept me sane and with the feeling I was part of the free world." That's right, boys and girls. Rock n' Roll defeated Communism.
Not just Communism – many people regarded John Lennon and Jimi Hendrix as anti-capitalist, anti-establishment revolutionaries.
Vaclav Havel, playwright and leader of the Czechoslovak anti-communists and first post-communist President of Czechoslovakia, was drawn into active resistance because of the Czechoslovak government's suppression of a band called the Plastic People of the Universe. He was also such a huge Zappa fan that he offered him a Cabinet post—but the US government, which didn't like Zappa after his testimony to Congress regarding content warning labels on recordings, intervened to prevent it. Most importantly, the revolution Havel led was the only completely non-violent one.
Of course, it was not just Havel, but a whole group of people involved in culture, some of whom were so old they probably never listened to rock in their life: but they all agreed that persecuting someone just because they liked a specific genre of music was the last straw after which they could not remain silent.
The awesomest part of the story: Plastic People of the Universe was a cover band; which band? The Velvet Underground - inspired by the first two albums which Havel had brought back from New York before the Soviet invasion. And what was the name of Havel's revolution in late '80s? The Velvet Revolution. Yeah.
Followed by the Velvet Divorce, which peacefully split the country into Slovakia and the Czech Republic.
The leading singers of Brazil's psych/prog-rock movement, Tropicalia, were inspired by the Beatles and other classic rock, but were censored and banned with threat of jail time by Brazil's then-oppressive government which declared the whole genre subversive because it inspired Brazil's youth. They also took inspiration, naturally, from their own native music.
Inverted by some store owners, who play classical music in their establishments' parking lots to discourage skateboarders or gang members from hanging around there.
According to "Wherever We Go", Newsboys music improves stock markets, reduces crime, and repairs the ozone layer, among other things.
Israeli progressive folk metal band, Orphaned Land, has a petition with over 2000 signatures nominating them for a Nobel Peace Prize because they step over otherwise deep-set lines of divide to reach out to people of all nationalities and faiths, as found here.YouTube comments left by fans from the Middle East often condemn the war and conflict that has been so common in the region lately.
In Times Square, protestors counter an anti-Islamic speech by pastor Terry Jones by singing the Beatles' All You Need Is Love.
Tuomas Holopainen saved himself from suicide by composing "The Poet and the Pendulum."
Scottish clans used to use the eerie sound of bagpipes to demoralize their enemies, especially the non-Gaelic ones who didn't know what the hell that sound was.