Astonishingly, in dribs and drabs, the music comes. Sometimes the contour of a phrase occurs to him before he has a hint of what the words themselves will be; sometimes the words call forth the cadence; sometimes the shade of a melody, having hovered for days on the edge of hearing, unfolds and blessedly reveals itself.
Joseph Hutchison, Colorado Poet Laureate 2014-2019, has published 20 collections of poems and edited or co-edited three poetry anthologies. He currently directs two master’s-level programs for University College at the University of Denver: Professional Creative Writing and Arts & Culture Management. Joe lives with his wife, Melody Madonna, in the mountains southwest of Denver, Colorado, the city where he was born. 
Yes! Like most things for Lurie, the music takes place only in his head. Even the horrific events he goes through in the novel don't significantly change his disconnection from reality—the reality in which he is no composer at all. "A crank" describes him perfectly.
What's odd about this is that David Lurie is not a composer but a bit of a crank. As so often in Coetzee, the beautiful bit is called into question by its context in the story.