The velocity of longitudinal ultrasound propagated along the a axis in URu2Si2 has been studied at temperatures down to 40 mK and in magnetic fields of up to 45 T applied along the c axis. The frequencies of the observed acoustic quantum oscillations (560 T and 1070 T), as well as calculated effective masses (14.5me and 22me), are in reasonable agreement with the known dHvA extremal cross sections of the Fermi surface in URu2Si2. It is shown that features observed previously at temperatures of about 0.5 K in magnetic fields between about 29 and 30 T in the magnetoresistance. Hall coefficient and ultrasound velocity are associated with quantum oscillations, which are clearly seen at lower temperatures. A hysteresis observed at 40 mK for all three steps of the metamagnetic transition confirms that they all are first order transitions.