What is Cloud Atlas?

Cloud Atlas, in addition to being the name of the novel, is the name of the concerto that Robert Frobisher composes in the "Letters from Zedelghem" chapters. Luisa Rey, in the subsequent chapter, searches for a copy of the work after reading about it in Frobisher's letters to Sixsmith. When she locates one of the few recordings available, she is surprised to recognize it in a deja-vu moment.