yes,
for natural gas mixtures I would select the Extended Peng Robinson or the Extended Soave Redlich Kwong versions available in PRODE PROPERTIES,
you may also consider BWR or (when water-alcohools as methanol etc.) present the different variants of CPA.

For calculating the speed of sound the procedure needs to evaluate density, enthalpy and entropy
so when selecting the model you should consider the accuracy
given by the different models for these properties.