The preaching itself, heard by each hearer in their own language.
The claim is not that the Tīrthaṅkara speaks many languages but that one sound is understood by everyone present, including the animals on the deck below.
It is the answer to a problem the seating raises: an assembly of gods, humans and beasts needs a speech that does not have to choose an audience.
The Digambara tradition holds that the sound is inarticulate and is rendered into words by the chief disciple. That is a real divergence and is marked as one.