“With whatever purpose you listen to music, that purpose will be increased.”

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.

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.

With whatever purpose you listen to music, that purpose will be increased. For instance: there will be a concert given for the poor and unfortunate, and if you go there thinking of the aim, the music will increase your compassion and generosity. This is the reason why music is used in war. And so it is with all the things that cause the excitation of the nerves. 

- 'Abdu'l-Bahá  (Utterances of 'Abdu'l-Baha to Mrs. Mary L. Lucas, as quoted in "A Brief Account of My Visit to Acca", Chicago: Bahá'í Publishing Society, 1905, pp. 11-14; The Compilation of Compilations, vol. II, Music)