We report the results of nuclear magnetic resonance imaging experiments on vertically vibrated granular beds of mustard grains. A novel spin-echo velocity pro. ling technique was developed that allows granular temperature, mean velocity and packing fraction distributions within the three-dimensional cell to be measured as a function of both vertical position and vibration phase. Bimodal velocity distributions were observed at certain portions of the vibration cycle, and in general the ability to acquire time-resolved data demonstrated the significant distortions to the velocity distributions and the systematic errors in calculated temperature distributions that may arise with time-averaged measurements. The experimental behaviour was compared with predictions from a time-varying one-dimensional hydrodynamic model using the experimental parameters as input to the code. In both cases, damping of longitudinal sound waves was linked to significant volume heating effects, which contrasts with the usual heat transport mechanism (i.e. diffusion from the boundaries) currently assumed in most steady-state models. This leads to a new explanation for the counterintuitive upturn in granular temperature in vibrofluidized granular beds, based on amplification and damping of sound waves in the high-altitude region.