Some types of music were suppressed
Why would anyone want to suppress music? These days, any parent can understand this question after listening to too many hours of their teenager's hard rock or rap. (Or the other way around, from the kids' perspective.) Fortunately for all of us, nobody is in a position to actually suppress any kind of music from being performed today - we just let the market take care of who listens and who doesn't.
But there were times in the past when certain kinds of music were banned. More specifically, certain intervals of two or more notes that sounded together or in succession were banned. In the Middle Ages, the Roman Catholic Church banned an interval that we would today write as the notes C and F#, which is called a tritone. In those times, musical instruments were tuned differently than they are today, and our modern instruments can't reproduce this interval exactly. (There are, however, other ways of tuning instruments, still extant today, that do reproduce this interval. The Mesopotamian tunings heard on Memories of Home, for example.) To hear it, or something very close to it, sing the first two notes of Leonard Bernstein's song "Maria" from West Side Story.
The Church called this interval "the devil's interval" or "the devil's chord", and disallowed it in any music. Since the Roman Catholic Church was in those days powerful enough - through fear, mostly - they effectively eliminated it from all subsequent European music.
But why would they do this? What possible harm could two musical notes be? A question more to the point would be, What possible threat could they be to the Church? Kay Gardner has summed up the answer in her excellent book, Sounding the Inner Landscape:
Personally, I feel that the tritone, when sung at length as harmony by a group of meditators, will take singers and listeners to a place where they will be in touch with Divinity. Perhaps this is a reason why it was so threatening in ages past (p. 117).
The role of the Roman Church
Now things become clearer. If ordinary people - just singing or even listening - could contact God (or the Creator, or whatever term you prefer), then who needed the Church? It is hard for us today to appreciate the power the Church wielded in all of Europe from the fourth to the early twentieth centuries. For most of this period, religion and politics were one and the same thing. The Roman Church, as it evolved from about 330 C.E. (Christian Era), was to have been the only path to God. This path was limited to the hierarchy that extended from the local parish priest all the way up to the bishop of Rome, who later took the title Pope. Religious control meant political control, and therefore economic control and educational control. During this period, the Church leaders were the thought police in everything but name. By 800 C.E. and the crowning of Charlemagne, the Church controlled every major political office on the Continent.
Anyone professing or causing anything that remotely promised a direct spiritual contact (or music that specifically delivered the possibility of such contact) was a threat to the Church, which was and had to be the only game in town. (The only game in every town.)
So it was not the musical notes or intervals that were suppressed in and of themselves, but the experience those notes could evoke. Even today, we are really just coming out of a lingering and very real modern Dark Age. Few people are aware that music can have such profound effects. But lots of people knew it a thousand years ago, and for millennia before that.
Since this is primarily a music site, not too much more will be said about other things that have been suppressed, some for centuries. But could we imagine a culture wherein everyone could have profound, moving experiences by simply listening to music? Where, by this very experience, the world we learned to perceive since earliest childhood would be much broader and richer? Would we find new meanings in the world for ourselves? Would there be more tolerance and compassion, since we would understand ourselves, and therefore, others, better?
Perhaps by playing a harmonically correct tritone -and the other ancient tuning intervals in modern music - we'd find out.