Trafalgar D. Water Law's inability to hear the voice of Zunesha, the giant elephant on the island of Zou in the anime and manga series "One Piece," has not been explicitly explained by the series' creator, Eiichiro Oda. Therefore, any explanation beyond that point is pure speculation, and I may not be aware of any updates or developments in the series that occurred .
At that time, it was implied that only those with the ability to hear the "Voice of All Things" could communicate with animals and inanimate objects, like Zunesha. Monkey D. Luffy is one of the few characters known to have this ability, which allowed him to communicate with Zunesha during his visit to Zou. Trafalgar Law, as far as we knew, did not possess this particular ability, which is why he couldn't hear Zunesha's voice.
For the most accurate and up-to-date information, I recommend checking the latest chapters or episodes of "One Piece" to see if any new developments have occurred regarding this matter.