In 1324, Pope John XXII issued a papal bull condemning the excesses of modern popular music.
Over the course of his papal career, John XXII had a few interesting battles. He was an early enemy of witchcraft, issuing a decree (papal bull) denouncing those who used magic. He excommunicated William of Ockham – who we know best as the namesake of Ockham’s Razor – over a dispute about the nature of Christ’s poverty. And, in 1324, he released Docta Sanctorum Patrum, which took aim at modern popular music.
The music of public Catholic worship – the liturgical chant – had been a particular way for hundreds of years. Plainsong, as it was called, was monophonic: many singers but just one melody at a time. During worship, the choir and the congregation alternated singing; the result we know as Gregorian chant:
Medieval music began to grow out of these monophonic origins to include more voices, either accompanying the melody (heterophony, e.g. the semi-improvised baseline of organum) or introducing new melodies (polyphony). In contrast to the traditional plainsong, polyphonic music is beautifully ornate, with voices interweaving and dancing around each other. It was also intensely controversial.
John XXII was an Avignon pope: his court was in France rather than Rome. This caused him some problems, such as when an excommunicated Holy Roman Emperor invaded Rome and installed his own antipope. (That particular antipope didn’t last for long.) It also meant that the papal court saw a lot of this new exciting polyphonic music creeping into religious contexts.
The pope was having none of this new-fangled “popular” music corrupting his church’s ancient traditions, so he issued the papal bull Docta Sanctorum Patrum to rail against it:
These composers, knowing nothing of the true foundations upon which they must build, are ignorant of the church modes, incapable of distinguishing between them, and cause great confusion. The great number of notes in their compositions conceals from us the plainchant melody, with its simple well-regulated rises and falls that indicate the character of the church mode. These musicians run without pausing. They intoxicate the ear without satisfying it; they dramatize the text with gestures; and, instead of promoting devotion, they prevent it by creating a sensuous and indecent atmosphere…
Pope John XXII, Papal Bull Teachings of the Holy Fathers (1324)
The indecency of polyphony! This pope’s battle against musical impropriety was lost before it began: forms with multiple melodies were already widespread, and within the century Guillaume de Machaut’s polyphonic mass Messe de Nostre Dame became one of the most influential pieces of religious music in Western history:
The Generalist Academy 
Nice collection of early music: https://www.youtube.com/user/Eric1Boul/videos