I came across a post theorizing that Law could have the Voice of All Things. Honestly, I've always had an inkling that he might from the events of Dressrosa (Wano threw me off a little, but there's still a chance imo!). However, what that post noted which I failed to do was to make the connection between that and and his motivation to carry out the Rocky Port Incident during the timeskip.
And honestly? It makes a lot of sense & I feel a little silly for not realizing it sooner! Until now, I found Law's pre-dressrosa interest in the poneglyphs to be kind of baffling. But if he *were* to have VOAT, then he would have the capability to at least discern their importance like Roger did. Hence his interest to investigate the one at Hachinosu.
That would then follow that the Hearts have come across at least one in their travels beforehand. The one at fishman island is a good contender ofc, but there could have been others in the north blue / grand line as well.