The propagation of ultrasound waves in thermoplastic multiblock elastomers was investigated by measurements of the ultrasound velocity and attenuation. Three polymer series with amorphous-crystalline structure were studied. The soft-segment material was composed of three different oligomers for the three series: poly(1,4-oxypropylene)diol for series A, oligo(oxyethylene)diol for series B, and dimerized fatty acid for series C. The hard-segment material was composed of poly(butylene terephthalate) (PBT). The ultrasound velocity and attenuation of these elastomers with variable hard-segment concentration were investigated as a function of the temperature. The observed relaxation processes were ascribed to the glass transition of the soft phase. The possibility of a general description of these systems is discussed.