Despite the fact that most porous materials, of influence in building and urban acoustics, are either fibrous or granular in nature, theoretical models used to predict their acoustical behavior at audio frequencies are derived from an assumed pore-based microstructure. This leads to the need for pore-related parameters that are either difficult to measure or not measurable independently. An alternative is to start from the known size and shape distribution and properties of the constituent solid particles. One possible particle-based approach is coupled phase theory. This has been used extensively to predict sound propagation in suspensions and emulsions. By means of an expression for the interphase inertial force appropriate to spherical particles, it has been shown that ``frameless'' Biot theory is equivalent to coupled phase theory modified to include hydrodynamic interactions between the forces acting on individual particles. The two models differ only in their expression of the interphase viscous force. Extensions of the coupled phase approach that are necessary when considering air-filled granular and fibrous materials are discussed.