Microporous membranes of high-density polyethylene were prepared by melt-extrusion followed by annealing and uniaxial extension. Crystallization at a high melt flow rate and subsequent annealing of the spun films with fixed ends led to the formation of oriented hard-elastic materials with a high modulus of elasticity and a considerable work recovery. Uniaxial stretching of such systems along the orientation direction induced the formation of microscopic pores due to the specific structure of the hard-elastic material. At some critical values of the processing parameters, throughflow channels were formed, converting the film into a microporous membrane permeable to liquids and vapors. Sound propagation, tensile measurements, and X-ray diffraction techniques were used to characterize the structure and properties of the samples at individual stages of the process as a function of the processing parameters. In particular, it was shown that polar diagrams of the sound propagation velocity reflect sensitively the structural changes in the process of porous structure formation.