Aqueous solutions of a PEO-PPO-PEO copolymer were studied using Rayleigh-Brillouin (R-B) and dynamic light scattering (DLS) over the concentration range 0.1-0.4 g/mL as a function of temperature. The purpose was to investigate associative and aggregation phenomena in the system. Although the R-B spectra were not influenced by the micellar to cubic phase transition, both the sound velocity (v) and spatial attenuation (alpha) increased with decreasing temperature below the phase transition temperature in the region in which micellar cluster formation is observed using DLS. The observations can be explained in terms of the structure of the solutions. The increase in v and alpha is shown to be caused by the scattering of sound waves by the clusters.