  The hydrocarbons seas of Titan, discovered by Cassini/Huygens mission are
among the most mysterious and interesting features of this moon. In the future,
a possible dedicated planetary probe, will certainly measure the speed of sound
in this cryogenic liquid, as it was planned in the case of a Huygens landing
into a sea. Previous theoretical studies of such acoustic measurements were
based on rather simple models, leading in some cases to unphysical situations.
Employed in a vast body of chemical engineering works, the state of the art
PC-SAFT model has been recently introduced in studies aimed at Titan. Here, I
revisit the issue of the speed of sound in Titan's liquids, in the light of
this theory. I describe, in detail, the derivation of the speed of sound from
the chosen equation of state and the potential limitations of the approach. To
make estimations of the composition of a ternary liquid mixture N2:CH4:C2H6
from speed of sound measurements an original inversion algorithm is proposed.
It is shown that 50 measures between 90 K and 100 K are enough to ensure an
accuracy of the derived compositions better than 10%. The influence of the
possible presence of propane is also investigated.
