We measured the sound velocity for magnetic soft composites consisting of a polymeric matrix of polyurethane and a magnetic particle of carbonyl iron or barium ferrite, and investigated the effect of the particle dispersibility on the sound velocity at 10 MHz for the composites. Morphological observations and dynamic viscoelastic measurements at 1 Hz were also carried out, and the results revealed that the carbonyl iron was dispersive particle and the barium ferrite was aggregative particle in the polyurethane matrix. The sound velocity for polyurethane elastomer without magnetic particles was 1444 +/- 12 m/s and it decreased with the volume fraction of magnetic particles. The sound velocity of magnetic composites with barium ferrite obeyed the Reuss model at whole volume fractions, however, magnetic composites with carbonyl iron exhibited an under deviation from the Reuss model at volume fractions above similar to 0.053. It was also found that the longitudinal modulus was insensitive to the creation of the particle network and sensitive to the aggregation of magnetic particles. The threshold seen in the longitudinal modulus was different from that in the nonlinear parameter of viscoelasticity, indicating that the ultrasound of 10 MHz detects microscopic heterogeneity of particle dispersion.