The attenuation ..cap alpha.., the velocity u, and the dispersion D = u (..omega..) - u (0) of first sound have been determined in /sup 3/He-/sup 4/He mixtures (molar /sup 3/He concentrations X/sub 3/ = 0, 0.070, 0.194, 0.377, and 0.517) at frequencies 2.3 T/sub lambda/, our absorption and dispersion data for all ..omega.. and X/sub 3/ can be scaled with functions of ..omega..tau over four decades of ..omega..tau. This scaling analysis shows that the time tau characterizing the critical attenuation and dispersion at T < T/sub lambda/ has the same temperature and concentration dependence as the relaxation time tau' at T < T/sub lambda/; these two times differ at most by a constant multiplicative factor. Below T/sub lambda/, the data are represented by the sum of the contribution represented by the scaling function plus the contribution from order parameter relaxation. (AIP)« less