The song that Firdaus hears has not been heard before, but its sentiments have been experienced before.  In the context of her remembering the song, Firdaus has recognized that she, once again, has been betrayed by a man for sex.  Once she recognizes that Ibrahim has used her for sex, she begins to understand that the life she led as a prostitute had more dignity for her as the life being led as a "respectable" woman.  Her experience with Ibrahim has convinced Firdaus that "All women are victims of deception," a reality that has been present in her experience before:

Men impose deception on women and punish them for being deceived, force them down to the lowest level and punish them for falling so low, binding them in marriage and then chastise them with menial service for life, or insults, or blows.

This is a condition that Fridaus has known before throughout her life.  Her experiences with Ibrahim have now taught her that "the least deluded of all women was the prostitute" because "respectability" through the institution of marriage "was the system built on the most cruel of suffering for women."

The song she hears is a song of liberation, of casting off constrictions to embrace what represents the truth.  For Firdaus, she has not heard the song explicitly, but its meaning has been understood in the light of her past experiences.  It is at this moment of hearing the song that she understands the freedom she desires exists in being a prostitute as it enables her to have some control.  In being able to shed the label of "social acceptance" and seeking to adhere to what is deemed as "respectable," she stands a better chance of finding control or freedom in her being.  The song is new to her, but its sentiments have been present throughout her life and are realized at that particular instant.