Music accentuates whatever feelings one has is in his/her heart

Whatever is in the heart of man, melody moves and awakens. If a heart full of good feelings and a pure voice are joined together, a great effect is produced. For instance: if there be love in the heart, through melody, it will increase until its intensity can scarcely be borne; but if bad thoughts are in the heart, such as hatred, it will increase and multiply. For instance: the music used in war awakens the desire for bloodshed. The meaning is that melody causes whatever feeling is in the heart to increase. 

- 'Abdu'l-Baha  (Words of ‘Abdu’l-Baha, recorded by Mary Lucas during her pilgrimage in 1905; ‘A Brief Account of My Visit to Acca’, by Mary Lucas)

Human feelings – some are accidental & some based on the real nature of the individual

Some feelings occur accidentally and some have a foundation. For example: some people are naturally kind, but they may be accidentally upset by a wave of anger. But if they hear music, the true nature will reassert itself. Music really awakens the real, natural nature, the individual essence. 

- 'Abdu'l-Baha  (Words of ‘Abdu’l-Baha, recorded by Mary Lucas during her pilgrimage in 1905; ‘A Brief Account of My Visit to Acca’, by Mary Lucas)

Human voice though physical has a connection with spirit

In the world of existence physical things have a connection with spiritual realities. One of these things is the voice, which connects itself with the spirit; and the spirit can be uplifted by this means—for though it is a physical thing, it is one of the material, natural organizations— therefore, it is effective. 

- 'Abdu'l-Baha  (Words of ‘Abdu’l-Baha, recorded by Mary Lucas during her pilgrimage in 1905; ‘A Brief Account of My Visit to Acca’, by Mary Lucas)

One’s spirit is naturally attracted to beautiful and graceful things

It is natural for the heart and spirit to take pleasure and enjoyment in all things that show forth symmetry, harmony, and perfection. For instance: a beautiful house, a well-designed garden, a symmetrical line, a graceful motion, a well written book, pleasing garments—in fact, all things that have in themselves grace or beauty are pleasing to the heart and spirit… 

- 'Abdu'l-Baha  (Words of ‘Abdu’l-Baha, recorded by Mary Lucas during her pilgrimage in 1905; ‘A Brief Account of My Visit to Acca’, by Mary Lucas)

A “conflagration, the like of which is not recorded in the past history of mankind”

A great melee of the civilized nations is in sight. A tremendous conflict is at hand. The world is at the threshold of a most tragic struggle.… Vast armies—millions of men—are being mobilized and stationed at their frontiers. They are being prepared for the fearful contest. The slightest friction will bring them into a terrific crash, and there will be a conflagration, the like of which is not recorded in the past history of mankind. 

- 'Abdu'l-Baha  (Remarks made in Haifa, August 3, 1914; ‘Baha’u’llah and the New Era’, by John Esselmont)