Plato felt that music was something very important in society, and even advocated banning certain music or art if it did not work for the society as a whole.
He also said: "the introduction of a new kind of music must be shunned as imperiling the whole state; since styles of music are never disturbed without affecting the most important political institutions." Socrates, in The Republic, bk. 4, sct. 424.
In Book 3 of the Republic, For music, only the kinds that are suitable for the depiction of “moderate and courageous men” wil be allowed (399c).
Plato/Socrates thought that by creating beautiful art only, then people would have more appreciation of beauty which will then have him love reason. Justice could then follow.
He feel that music and justice is related. If people love art more, they would love music too, which leads to love more reasoning too, which is required for justice to happen