  We present a simple approach to extract hydrodynamic sound modes and
non-hydrodynamic modes in kinetic theories from response functions of the
energy-momentum tensor. By comparing the response functions in four types of
kinetic theories, namely the Relaxation-Time Approximation, scalar $\phi^4$
theory, SU(3) Yang-Mills theory and QCD kinetic theory, we find a remarkable
degree of universality for the sound mode, even beyond the hydrodynamic regime.
