The dual function exhaust system attachment, which is preferably used with an all-terrain vehicle, includes a muffler, a silencing cap, and a sound-enhancing plug. In a quiet configuration, the silencing cap is mounted to conceal a primary outlet of the muffler. In a loud configuration, the sound-enhancing plug is mounted to conceal a secondary outlet of the muffler. A set of screw threads or sealing mechanism may be used to mount the silencing cap and the sound-enhancing plug. A primary perforated barrier, a solid barrier, at least one secondary perforated barrier, and a pipe section are positioned within a hollow structural body of the muffler to guide the exhaust gas flow from an inlet of the muffler to the primary outlet or the secondary outlet.