The Republic (Greek: Πολιτεία, Politeia) is a Socratic dialogue written by Plato around 380 BC concerning the definition of justice and the order and character of the just city-state and the just man (from Wikipedia).
In other words, The Republic of Plato is the perfect state as imagined by Greek philosophers. The book is definitely a recommended reading.
Here are a few paragraphs discussing the effect of music on the guardians of the state. The book is mainly in form of a discussion between philosophers.
“This is the sort of medicine and this the sort of law which will prevail in our State; they will be healing arts to better natures; but the evil body will be left to die by the one, and the evil soul will be put to death by the other. And the need of either will be greatly diminished by good music which will give harmony to the soul, and good gymnastic which will give health to the body. Not that this division of music and gymnastic really corresponds to soul and body; for they are both equally concerned with the soul, which is tamed by the one and aroused and sustained by the other. The two together supply our guardians with their twofold nature. The passionate disposition when it has too much gymnastic is hardened and brutalized, the gentle or philosophic temper which has too much music becomes enervated. While a man is allowing music to pour like water through the funnel of his ears, the edge of his soul gradually wears away, and the passionate or spirited element is melted out of him. Too little spirit is easily exhausted; too much quickly passes into nervous irritability. So, again, the athlete by feeding and raining has his courage doubled, but he soon grows stupid; he is like a wild beast, ready to do everything by blows and nothing by counsel or policy. There are two principles in man, reason and passion, and to these, not to the soul and body, the two arts of music and gymnastic correspond. He who mingles them in harmonious concord is the true musician,–he shall be the presiding genius of our State.”
“And therefore, I said, Glaucon, musical training is a more potent instrument than any other, because rhythm and harmony find their way into the inward places of the soul, on which they mightily fasten, imparting grace, and making the soul of him who is rightly educated graceful, or of him who is ill-educated ungraceful; and also because he who has received this true education of the inner being will most shrewdly perceive omissions or faults in art and nature, and with a true taste, while he praises and rejoices over and receives into his soul the good, and becomes noble and good, he will justly blame and hate the bad, now in the days of his youth, even before he is able to know the reason why; and when reason comes he will recognise and salute the friend with whom his education has made him long familiar.”
“And when they have made a good beginning in play, and by the help of music have gained the habit of good order, then this habit of order, in a manner how unlike the lawless play of the others! will accompany them in all their actions and be a principle of growth to them, and if there be any fallen places in the State will raise them up again.”
“In like manner the poet with his words and phrases may be said to lay on the colours of the several arts, himself understanding their nature only enough to imitate them; and other people, who are as ignorant as he is, and judge only from his words, imagine that if he speaks of cobbling, or of military tactics, or of anything else, in metre and harmony and rhythm, he speaks very well–such is the sweet influence which melody and rhythm by nature have. And I think that you must have observed again and again what a poor appearance the tales of poets make when stripped of the colours which music puts upon them, and recited in simple prose.”