Blogger’s note: Pay particular attention to the fact that Khan uses the term CULTIVATED when referencing the development of the voice, not PRODUCTION. This may be a strong suggestion that he prefers the more agrarian view of voice development. 

There is another point which is most wonderful about the voice: that once you have worked with the voice and have cultivated it, deepened it, widened it, and it has become invigorated, and then you have left it, you may leave it for months and years, and the voice may take a different shape and a different appearance, but at the same time what you have once developed remains with you somewhere. It is just like a kind of deposit kept in a bank. You do not know of it, you have forgotten it perhaps, yet it is there. The day when you will touch it again, it will come back in the same way and it will take very little to complete it.

If the voice has developed a spiritual quality and one finds later that it has lost that spiritual quality, one must not be discouraged or disappointed. One has not lost it. One must correct oneself and want to go forward again, and be sorry for having gone backward, but never be discouraged, never be hopeless, because it is there; it only wants a little touch. It is just like a little candle which has gone out, but once you strike a match it is lighted again; it is a candle just the same. The voice is light itself. If the light has become dim, it has not gone out, it is there. It is the same with the voice. If it does not shine, it only means that it has not been cultivated. You must cultivate it again, and it will begin to shine again.

Khan, Hazrat Inayat. The Mysticism of Sound and Music: Revised Edition. Shambhala Publications. 2014.