On p.108, Marlow speaks of Kurtz as "a voice." Soon thereafter, how does Marlow's manner of relating his story change? What seems to be the reason for his fascination with Kurtz' voice?

Marlow becomes obsessed with Kurtz's voice, "The man presented himself as a voice." Marlow even breaks the narrative to explain he gets to talk to this "voice". Marlow seems to go on about voices, his voice Kurtz's voice...Is this what Marlow considers the soul of a man?