  For any physical system satisfying the Einstein's equations, the comoving
curvature perturbations satisfy an equation involving the momentum-dependent
effective sound speed, valid for any system with a well defined energy-stress
tensor, including multi-fields models of inflation. We derive a general
model-independent formula for the effective sound speed of comoving adiabatic
perturbations, valid for a generic field-space metric, without assuming any
approximation to integrate out entropy perturbations, but expressing the
momentum-dependent effective sound speed in terms of the components of the
total energy-stress tensor. As an application, we study a number of two-field
models with a kinetic coupling between the fields, identifying the single
curvature mode of the effective theory and showing that momentum-dependent
effective sound speed fully accounts for the predictions for the power spectrum
of curvature perturbations. Our results show that the momentum-dependent
effective sound speed is a convenient scheme for describing all inflationary
models that admit a single-field effective theory, including the effects of
entropy perturbations present in multi-fields systems.
