In 1936, L. I. Mandel'shtam, together with M. A. Leontovich, extended the concept of Kneser-type relaxation to liquid media, and at the same time introduced the notion of microscopically inhomogeneous acoustical media and of another type of relaxation appearing in such media (``nonlocal relaxation''). These ideas were used (first at the suggestion of L. I. Mandel'shtam) for studying the acoustical behavior of such media as suspensions, emulsions, polycrystals, liquids with gaseous bubbles, etc. At the present time the same picture is used for constructing a theory of the sound propagation in pure high-viscosity liquids (the behavior of which does not fit into the scheme of Kneser-type relaxations) and for clarifying their structure. It has been established that the existing experimental material concerning the propagation of different types of waves in a number of high-viscosity liquids agrees well with the notion of a high-viscosity liquid represented by a microscopically inhomogeneous medium with structure: a disordered liquid with ordered regions (``clusters''). The assumed applicability of the picture of a microscopically inhomogeneous medium to high-viscosity liquids and nonlocal relaxation is likewise supported by observed changes in the values of a number of parameters for such liquids as a result of distillation and the gradual return of these parameters to their initial values.