As Kurtz lies dying from his illness, he gives long speeches about his life and his opinions. His body can no longer perform his will, but his best talent -- his eloquent speaking style -- remains almost to the last.
Kurtz discoursed. A voice! a voice! It rang deep to the very last. It survived his strength to hide in the magnificent folds of eloquence the barren darkness of his heart.
(Conrad, Heart of Darkness, eNotes eText)
Marlow sees that Kurtz is so damaged by the jungle and his actions that his body has given out; Kurtz has nothing left to offer the world except the contents of his mind, which he is slowly losing. In an effort to leave one last mark on the world, he speaks constantly, letting loose every aspect of his mind regardless of the opinion of others. Kurtz's voice is the last aspect of himself that has survived his descent into the "heart of darkness," a descent that has even overpowered his own inner heart and soul.