Kundera makes clear his metaphor of musical composition for a lifetime, and several times he has reinforced the image if not outright making it a clear statement.
My own inclination is to often call all creative forces "art" and so as a metaphor for the way one composes his life would be to also call it writing the pages, or painting the picture. Kundera, by using the art of music, manages to bring in movement as he brings the characters of Sabina and Franz into their separate experience with parades.
And so as long as he lived in Paris, he took part in every possible demonstration. How nice it was to celebrate something, demand something, protest against something; to be out in the open, to be with others. (…) The march from revolution to revolution, from struggle to struggle, ever onward. (p. 99)
Kundera here uses the march and its essential accompaniment of music to focus changes of more than character, to approach changes of a place–a country and its society–writing its historical composition. Kundera then goes on to state what he seems to think of the metaphor or writing for life:
Franz felt his book life to be unreal. He yearned for real life, for the touch of people walking side by side with him, for their shouts. It never occurred to him that what he considered unreal (the work he did in the solitude of the office or library) was in fact his real life, whereas the parades he imagined to be reality were nothing but theater, dance, carnival–in other words, a dream. (p. 100)
What is Kundera saying, exactly? Even as he makes the case for the movement and physical reality of the crowds, he places Franz in a position of not realizing that his reality was in truth his solitary office and library life. The parades Kundera calls, a dream; why? Because they are transitory? Because they are bolder and more exciting than the reality? Is a concert more or less real than a novel?
The Lost Children: A Charity Anthology