The propagation of elastic perturbations in magneto-rheological suspensions is studied theoretically and experimentally. Under the application of a magnetic field, these systems acquire a fibrillar fractal structure formed by clusters. In systems in that condition, two low-frequency sound propagation modes have been observed. In both of them, the speed of sound depends on the intensity of the applied field. We discuss the statistical fractal properties of the cluster structure and, on this basis, we calculate the speed of sound for both of the low-frequency modes. This theoretical approach provides a good quantitative agreement with the experimental results.