For the 2,6-dimethylpyridine-water mixture of critical composition broadband ultrasonic spectra and acoustical scaling function measurements are analyzed using relaxation rates of concentration fluctuations as resulting from dynamic light scattering and shear viscosity data. The Bhattacharjee-Ferrell dynamic scaling theory represents the experimental data adequately. The adiabatic coupling constant of this theory, calculated from the amplitude in the sonic attenuation spectra and the specific heat at constant pressure, is as small as g = 0.17, in conformity with a thermodynamic relation. No indications of interferences of the critical dynamics with an elementary chemical equilibrium are found. (c) 2006 Elsevier B.V. All rights reserved.