This chapter is delayed because I had to read it twice.
It's interesting to me how the protagonist does not hear any music. A lot of this book seems to be about art (the protagonist is a singer, the Marble Lady is a physical embodiment of sculpting, etc), so of course music is central to the conceptual framework here.
Note that in later chapters, the central hall is referred to as the Hall of Phantasy. In my opinion this hall is like some sort of encapsulation of the creative process, like a manifestation of artistic or creative inspiration.
The protagonist compares the Hall of Phantasy with the marble cave where he found the Marble Lady, and this is indicative of the similarities (though the Hall of Phantasy is mucher larger in scope). In the cave, he thought about Pygmalion and his sculpture, and he himself was inspired to sing a song that awakened the Marble Lady.
In the Hall, the protagonist is struck with wave upon wave of creative inspiration, where it be a drama, an epic tale, a song, beautiful images, etc.
I believe the reason why the protagonist had to surprise the statues is that his discovery of their motion also parallels the process of creative inspiration. That's why they could anticipate him entering their hall and even anticipate him *thinking* of entering their hall, but are caught off-guard when he does it with a sudden thought to enter, because he was essentially inspired and that allowed him to enter into the realm of creative magic. That magic took the marble statues from the stillness and death of mechanical life, of normality (what is the opposite of inspiration if it is not unthinking repetition?) and drew them into a rhythmic dance guided by celestial music that is not heard by human ears (and yet is discerned in the spirit).
Another similarity with the marble cave, is that the protagonist must sing another song to free the Marble Lady from this new tomb; rather than a tomb of alabaster, it is now a tomb of shadow. In the first case, it was like an unborn creation (from the sculpting metaphor, it was an unchiseled block of marble, yet with the perfect shape waiting within to be unleashed). In the second case, it was a stifled or smothered creation, destroyed or hidden by the sin and darkness that came from the heart of the protagonist (who is also the singer == creator). Of course it is the author's shadow who covers and smothers the creative power that comes from his heart. It is the shadow that battles against him finding and reaching his creative ideal and therefore it is his shadow that he must battle to free his Marble Lady. This seems to me like every artist's battle, to overcome the darkness in their life, to be free in their inspiration and to find and create that Marble Lady that we all seek.