Universality of sound modes in kinetic theory

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.