The temperature dependencies of ultrasonic velocity and attenuation were measured in composites of inorganic nanoparticles with two types of polymers, poly(urea) elastomer with inorganic Mo6S4I6 nanowires and poly(ϵ-caprolactone) (PCL) with Mo6S3I6 nanowires. Below room temperature large ultrasonic relaxation attenuation maxima and velocity dispersion were observed. It was found that the attenuation peak in the elastomer shifted to higher temperature after doping with nanoparticles and this behavior was related to the shift of glass transition temperature. The ultrasonic attenuation data was fitted to a relaxation equation with a single temperature dependent relaxation time. The thermal activation energy of the relaxation process, which was calculated from ultrasonic data, was found to increase in the poly(urea) elastomer doped with MoSI nanowires. The low temperature ultrasonic velocity increased in the poly(urea) with nanowires added and is determined by the increase in elastic modulus. Similar ultrasonic behavior was obtained for PCL composites with inorganic MoSI nanowires. In this case, the increase in elastic modulus was smaller in comparison to the composites of poly(urea) and nanowires. Therefore, reinforcement of PCL was less pronounced.