The theory of sound refraction by shear layers, established two decades ago, is revisited from the viewpoint of sound-vortex interaction: vortical waves are generated by sound on the duct rigid wall through viscosity, and these acoustically created vortical waves in turn serve as a source of sound. This viewpoint leads to an approach different from conventional ones. For a quadratic basic velocity profile at low Mach number, results are given from small to large wavenumbers with different Reynolds numbers, and compared with two simplified asymptotic approaches at the large wavenumber end. It is found that, as the wavenumber increases at a fixed Reynolds number, the resultant sound pressure level is first lower but then higher than corresponding inviscid values. Similar non-monotonic behavior for varying Reynolds numbers at fixed wavenumber is also observed. The importance of viscosity in sound production by vortex is confirmed, and the possible physical reason for sound amplification at large wavenumber is discussed. Unlike conventional methods in duct acoustics, the approach adopted here can be generalized to much wider sound-vortex interaction problems without fundamental difficulty.