  In this paper, a new general formula for the sound speed in adiabatic
conditions ( S = const ) has been established. The sound speed depends on the
mass density {\rho} (p,T ) and the internal energy per unit mass E(p,T ), both
expressed as functions of the pressure p and the temperature T . This formula
has been compared with experimental data on the example of triolein over the
pressure range up to 450 MPa. For experimental data, phenomenological
approximate formulas have been proposed. Those formulas have two versions,
depending on the 2 and 3 parameters. Both versions have been developed with the
help of the new expression (Eq.8) for the sound speed. The explicit form of
both approximate curves can be regarded as the result of purely
phenomenological modeling. However, in this paper, these new analytical
expressions have been obtained by applying the heuristic procedure described in
Appendix.
