  We discuss a field-theoretical approach to liquids, solids and glasses,
published recently [Phys.Rev.E {\bf105}, 034108 (2022)], which aims to describe
these materials in a common quantum formalism. We argue that such quantum
formalism is not applicable to classical liquids, and the results presented,
which rely heavily on the concept of phase relaxation borrowed from quantum
fluids, contradict the known hydrodynamic theory of classical liquids. In
particular, the authors miss the important particle-number conservation law and
the density fluctuations as hydrodynamic slow variable. Instead, the authors
invoke Goldstone bosons as elementary hydrodynamic excitations. We point out
that in a classical liquid there are no broken continuous symmetries and
consequently no Goldstone bosons. The authors claim that the Goldstone bosons
would be responsible for the existence of sound in liquids, instead of
resulting from combined particle-number and momentum conservation, a fact well
documented in fluid-mechanics textbooks.
