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Hydraulic hybrid propulsion systems are an acceptable solution for actual requirements imposed to vehicle propulsion systems, due to their flexibility and power density using a conventional technology. But hydraulic hybrids need still to be improved in order to have higher efficiencies. Sonic systems based on waves propagated in liquids are a solution due to their low flow speeds. The direct effects are high efficiencies of energy transfer. A problem of sonic systems is their frequency dependence that has to be correlated with the running conditions. In order to overcome this aspect hydraulic energy generation and effective propulsion process are decoupled. The last one is based on conventional hydraulic propulsion. Hydraulic energy is generated with sonic support and energy efflux to wheels is controlled by the hydraulic. This solution allows optimizing the hydraulic energy generation process. In order to demonstrate these facts, simulation and experimental results were analysed for an ATV vehicle.