IIRC, Huygens picked up an ELF reflection during descent that might have been from a deep solid-liquid interface. I don't recall hearing any more about that, though.
The way Cassini deflects as it flies by Titan will give some information about the radial density gradient within Titan, which will in turn set physical limits on what is or is not plausible about its internal structure. But I haven't heard any results from that, either.

Grant Hutchison