In this paper it is shown that our volume visco-elastic theory of fluids is applicable not merely to the case of structural relaxation as apparently regarded by some authors, but is equallyapplicable to all three kinds of relaxations-thermal, structural and chemical. From our equationof irreversibility of volume change (4), is inferred the equation of thermal irreversibility (10) as originally assumed by Herzfeld and Rice for the case of thermal relaxation. Also, in the case of chemical relaxation, our equation of irreversibility of volume change is shown to imply the equation of chemical irreversibility (15) as obtained from kinetic considerations by Lieber-mann.Regarding application of this theory to sound absorption and dispersion phenomena, Bourgin-Kneser equations (29) and (33) for the case of thermal relaxation and Liebermann equation (43) for the case of chemical relaxation are shown to follow directly from the results of our theory of compressibilities when the appropriate thermodynamic considerations have been made. This derivation reveals that Liebermann equation of sound absorption can be a good approximation only for the case of liquids.It is pointed out in passing that the accuracy of certain existing sound absorption and dispersion measurements in the case of gases has already made it possible to determine from them the static and instantaneous compressibilities β0 and β∞ and hence the two ratios of heat capacities γ0 and γ∞ and the external and internal heat capacities C(e) and C(i), and thus to draw directly from them certain conclusions regarding the structure of the molecule and the processes of energy exchanges upon collisions.Finally, the appropriateness of our definition of volume viscosity is discussed.