The first carbon-film sound velocities obtained with the piezoelectric method are given. The a-C:H films were deposited from methane using rf plasma chemical vapor deposition at different substrate biases and thus contain varying hydrogen concentrations. Measurements of density allowed the Young's modulii of the films to be calculated. Both the sound velocity and the Young's modulus reached a maximum as the substrate bias changed from -47 to - 175 V. The film with the maximal properties occurred at a bias of -76 V and had a sound velocity of 16.4 km/s, a Young's modulus of 589 GPa, an optical gap of 2.16 eV and a density of 2.19 g/cm(3). Although the modulus is 52% that of the directionally averaged value of diamond, it produced a sound velocity 91% that of diamond due to the lower film density. The film densities were in the range of 1.81-2.43 g/cm(3) with the densest films occurring at the highest bias magnitudes. Optical gap measurements were taken with a photo spectrometer and yielded a gap that decreased from 3.86 to 1.40 eV with increasing bias magnitude and thus decreasing hydrogen content. These values indicated sp.; concentrations that increased with the gap from 43% to 94% and these were consistent with Raman spectroscopy results.