Acoustic attenuation and phase velocity in the frequency range 2-50 MHz have been measured in a series of silica sols and gels with particle sizes in the range 12-30 nm, and concentrations in the range 5-40% (w/w). Results have been compared with both scattering and hydrodynamic models of acoustic propagation in colloids. Differences between measured and simulated results indicate that present models are inadequate for very small particle sizes and small particle separations (<50 nm), where very high number concentrations of scatterers are present.