  A model of nonlinear elastic medium with internal structure is considered.
The medium is assumed to contain cavities, microcracks or blotches of
substances that differ sharply in physical properties from the base material.
To describe the wave processes in such a medium, the averaged values of
physical fields are used. This leads to nonlinear evolutionary PDEs, differing
from the classical balance equations. The system under consideration possesses
a family of invariant soliton-like solutions. These solutions are shown to be
spectrally stable under certain restrictions on the parameters.
