The Symphony is the celestials' word for creation and everything in it. Angels try to live in harmony with the Symphony and use their powers by listening to it carefully; demons try to drown it out with their own individual tunes, pushing what they want onto the rest of reality. A human sorcerer often can't hear the "music" clearly and will refer to the "Cacophony" instead.
The musical metaphor also leads to many celestials calling themselves "instruments." See also dissonance, Discord, attunements and Songs for more examples.