  We investigate the response of the near-equilibrium quark-gluon plasma (QGP)
to perturbation at non-hydrodynamic gradients. We propose a conceivable
scenario under which sound mode continues to dominate the medium response in
this regime. Such a scenario has been observed experimentally for various
liquids and liquid metals. We further show this extended hydrodynamic regime
(EHR) indeed exists for both the weakly-coupled kinetic equation in the
relaxation time approximation (RTA) and the strongly-coupled ${\cal N}=4$
supersymmetric Yang-Mills (SYM) theory. We construct a simple but nontrivial
extension of M{\"u}eller-Isareal-Stewart (MIS) theory, namely MIS*, and
demonstrate that it describes EHR response for both RTA and SYM theory. This
indicates that MIS* equations can potentially be employed to search for QGP EHR
via heavy-ion collisions.
