Over recent years there has been great efforts towards the understanding of the rheology of immiscible polymer blends. It is now well accepted that their morphology, especially the concentration of the inclusions, their size, and their radii distribution, is a very important factor which controls their mechanical properties. The type of blend considered in this study is a polyamide/polypropylene system, in which the matrix is the polyamide (PA6) with inclusions of polypropylene (PP). Different concentrations have been used, as well as different surfactants. These blends give then rise to different acoustic properties, which have been characterized by measuring velocities of propagation and attenuation of ultrasonic waves during flow through a capillary rheometer. A wave propagation theory for viscoelastic emulsions was used to predict the values of the ultrasonic parameters as functions of the concentration, the radii distribution and the frequency when the thermophysical properties of the blend are known. We can therefore deduce the concentration and average size of inclusions, and then return to the morphology of the blends. Comparison with microscopic photographs seems to correlate well with our predictions. This method appears promising and could be used to differentiate between different blends during flow.