The speed of sound was computed via Monte Carlo simulation for a binary mixture of methane and n-butane in the subcritical, near-critical, and supercritical regions at the temperature of 311 K. The technique encompasses a sequential implementation of the isobaric isothermal and canonical ensembles in a simulation box, in which the (residual) thermodynamic derivative properties are evaluated via the fluctuation method [Escobedo, F. A. J. Chem. Phys. 1998, 108, 8761] during the Monte Carlo moves. A united atom Lennard-Jones potential with parameters proposed by Moller et al. [Moller, D. et al. Mol. Phys. 1992, 75, 363] was chosen to represent methane. In case of n-butane, we employed an optimized anisotropic united atom intermolecular Lennard-Jones description of Ungerer et al. [Ungerer, P. et al. J. Chem. Phys., 2000, 112, 5499]. In decent agreement with the experiment, we find the technique to provide reasonably quantitative estimates for the speed of sound in the thermodynamic regions studied.