Imagine yourself in a tunnel. It ECHOES!! Right? When Christian churches first built those huge cathedrals, the hard mosaic floors, stone walls and marble columns caused the same kind of echo. Remember, there was no electricity back then, so there no microphones or sound systems to help focus the sound to the listener. So when the priest spoke, his voice echoed and it made it hard to hear.
Digress with me a moment... Remember in sixth grade when you came to the stage & we looked at the science of musical instruments (sixth graders - this is a preview of what is coming later in your science of sound unit!)? One of the things we talked about was "sympathetic vibrations". Remember I hummed into the piano & it hummed back at me? And then we hit the timpani and the snare drums rattled? Or back in your science classroom your teacher set up two tuning forks, hit one and the other vibrated, too? Those are all examples of sympathetic vibrations. Now, back to our monks...
Well, the monks discovered that there was a "sympathetic note" - one pitch that was reinforced by the structure of the cathedral - that would sound more loudly than others. If the speaker chanted the words on that note and let his voice go down in a musical cadence, the sound would flow without an echo! Thus we have the beginning of Gregorian Chant (also called Plainsong) which was the music of the Christian Church for many, many years.