The sound generated by two square cylinders placed in a side-by-side arrangement in a uniform flow at low Mach numbers is studied by direct solution of the two-dimensional unsteady compressible Navier-Stokes equations. Special attention is paid to the effect of the spacing between the two cylinders on the generation mechanism of the sound. Results show that sound pressure fields as well as flow fields show different features depending on the spacing; six different wake patterns (nonsynchronized, antiphase and in-phase synchronized, flip-flopping, single bluff-body, and steady patterns) are observed and, depending on the wake pattern, sound pressure fields also show different features. Two types of bifurcation phenomenon are observed where two different wake patterns exist for the same spacing and either of the two patterns appears depending on an initial condition; different sound pressure fields appear in response to the wake patterns. Results also show that Curle's dipole solution does not predict well the direct numerical simulation results in the antiphase-synchronized pattern at a small spacing and the flip-flopping pattern, but that it gives a good approximation in the nonsynchronized, in-phase-synchronized, and single-bluff-body patterns.