Okay, so in episode four what I thought happened was that Hyakkimaru lost his leg but he still killed the antlion demon and got his voice back. But then in the next episode it's still alive, but he did kill it and got that leg back? So where did his voice come from? And if it hadn't eaten his leg what would he have gotten?
How I interpreted it is that his initial injury to the antlion that caused it to retreat counted as stealing his vocal chords; possibly, the demons don't actually need to be killed to retake the parts, but it's generally more efficient to do that and Hyakkimaru just got lucky in that regard this time. As for his leg regrowing, he probably gets his current body repaired whenever he gets a body part back, which could explain why he was able to keep going without rest; every body part he reclaimed refreshed him.
