Polybius, the judicious Polybius, tells us that it took music to tame the ways of the Arcadians, who lived in a land where the air is dreary and cold ; that the Cynetes, who neglected music, surpassed all the Greeks in cruelty, and that there is no city which has seen so many crimes.  Plato does not shrink from saying that no change can be made in music which is not a change in the constitution of the state. Aristotle, who seems to have written his Politics only to counter Plato’s opinions with his own, nevertheless agrees with him on the power of music over behavior. Theophrastes, Plutarch,  and all the Ancients were of the same mind. This is not an opinion hazarded without reflection : it is one of the principles of their politics.  That is how they gave laws, and that is how they wanted the cities to be governed.
I believe I could explain this. We must put ourselves in mind that in the Greek cities, especially those whose principal objective was war, all kinds of work and all the professions that could lead to earning money were regarded as unworthy of a free man. “Most of the arts,” says Xenophon, “corrupt the body of those who practice them ; they require one to sit in the dark or near the fire. One has no time either for friends or for the republic.”  It was only during the corruption of some democracies that artisans achieved the status of citizens. That is what Aristotle tells us,  and he maintains that a good republic will never accept them. 
Agriculture was another servile profession, and it was ordinarily some vanquished people that practiced it : the Helotes in Lacedæmon, the Perioeci in Crete, the Penestæ in Thessaly, and other enslaved peoples in other republics. 
In short, all base commerce  was repugnant to the Greeks. A citizen would have had to render services to a slave, a tenant, or a foreigner. Such a thought offended the spirit of Greek freedom ; thus Plato, in his Laws, would have any citizen who traded punished. 
So the situation was quite awkward in the Greek republics. They did not want citizens working in commerce, in agriculture, or in the arts, nor did they want them to be idle.  They found an occupation in exercises deriving from gymnastics, and in those related to war.  The institution offered them no others. The Greeks must therefore be regarded as a society of athletes and combatants. But those exercises that served so well to make people hardy and fierce needed tempering by others that could tame their behavior.  Music, which is allied to the mind through the organs of the body, was most appropriate for that. It is a medium between the exercises of the body that make men tough and the sciences of speculation that make them unsociable. We cannot say that music inspired virtue : that would be inconceivable ; but it prevented the effect of the ferocity of the institution, and gave the mind a role in education which it would not have had.
Suppose there were in our midst a society of persons so enamored of hunting that they did nothing else : it is certain that they would contract from it a certain ruggedness. If these same persons should also acquire a taste for music, a difference would soon be seen in their manners and ethos. In short, Greek exercises excited in them only one kind of passions : toughness, rage, and cruelty. Music excites them all, and can communicate gentleness, pity, affection, and gentle pleasure to the soul. Our moralists who so strongly condemn our theatres are proof enough of the power which music has over our souls.
If the society of which I have spoken were to receive only drums and trumpet airs, is it not true that the goal would be less likely attained than if they were given mellower music ? Therefore the Ancients were right when, in certain circumstances, they chose for the ethos one mode over another.
But, one will say, why single out music ? Because of all the pleasures of the senses, there is none that less corrupts the soul. We blush to read in Plutarch  that the Thebans, to tame the ways of their youth, used their laws to establish a kind of love that ought to be banned in every nation on earth.