The present work aims to measure speeds of sound c in a biogas mixture of CH4 + N-2 + CO2 + CO, at p = (1-12) MPa and T = (273, 300 and 325) K, using a spherical acoustical resonator. The results are fitted to the virial acoustic equation of state, and the virial acoustic coefficients are obtained, beta(a) and gamma(a) and extrapolated to zero pressure, determining the adiabatic coefficient as perfect gas, gamma(pg), and the isobaric and isochoric heat capacities as perfect gas, C-p(pg) and C-V(pg), respectively. The speeds of sound are acquired with a mean expanded relative uncertainty of 165 parts in 10(6) (k = 2) and are compared with the results predicted by the reference equation of state for this kind of mixture (natural gas-like mixtures), EoS GERG-2008. Relative deviations between experimental data and values estimated by this model were less than 700 parts in 10(6) at T = 325 K, and below 400 parts in 10(6), and within measurement uncertainty of at T = 300 K, although appreciably higher at isotherm T = 273 K at the highest pressure data for this work, and even reaching values above 3,400 parts in 10(6). (C) 2016 Elsevier Ltd.