I would speculate that an afterburner increases the noise because it is nearer to the exhaust. But it certainly causes so many changes in the exhaust flow that it could be less loud as well. It may also depend on the shape of the nozzle.
That an afterburner increases the spacial volume of exhaust gases does not necessarily mean that the sound volume increases as well, because it strongly depends on the level of turbulence.
Does activating an afterburner make a jet engine louder? And why?