The results of acoustic investigations of the viscous properties of a nematic liquid crystal mixture with wide temperature range at varying PVT-state parameters are presented. The original ultrasonic set-up was elaborated to provide the determination of some dissipative parameters including the ratio of twist viscosity gamma(1) to magnetic susceptibility anisotropy Deltachi as a function, of pressure (P) and temperature (T) at constant volume (V). It is found that the ratio gamma(1)/Deltachi exponentially decreases with increasing temperature or with decreasing pressure. This agrees with the free volume theory. According to our calculation free volume increases with temperature or decreasing pressure. The energies of activation (E-v, E-p) of the twist viscosity at constant volume and pressure are calculated. The dependence of Leslie viscosity coefficient alpha(1) on temperature is determined too.