Sound absorption in carbon dioxide with temperature
The acoustic wave is accompanied by a temperature oscillation. To a first approximation, the temperature oscillation is adiabatic and reversible in the bulk, but irreversible near solid boundaries. In many cases, boundary losses dominate (for example, in musical wind instruments), but as the frequency increases, and the surface-to-volume ratio decreases, bulk irreversibility may become important. Bulk losses in single-component polyatomic gasses are almost always due to loss of local thermodynamic equilibrium when the temperature oscillation describing internal degrees of freedom lags the temperature oscillation describing translational degrees of freedom.
A full discussion of these effects is given in the book &quot;Ultrasonic Absorption&quot; by A.B. Bhatia. His figure 5.1 shows that the attenuation for $\ce{CO2}$ peaks at about 30 kHz (STP conditions), and his figure 6.3 shows the velocity dispersion that occurs at about the same frequency. The dispersion is a large effect, with the velocity increasing by about 4% as the frequency goes from below to above the attenuation peak. The attenuation and dispersion occur when the acoustic frequency matches the rate at which the vibrational mode temperature relaxes to the translational temperature. The rotational modes relax much faster than the vibrational modes, so they do not play an important role under these conditions.
The relaxation time for the vibrational temperature will generally decrease with increasing temperature, so the attenuation peak will move to higher frequency. I am not sure what temperatures you are exploring, but if your frequency is above the attenuation peak, then you will see increasing attenuation as the temperature increases.