Dense mineral fillers are used to provide sound insulation properties to polymer-matrix composites. According to the mass law of sound insulation, the greater the mass, the harder it is for the material to vibrate, indicating the material will have better insulation. Thus, magnetite and barite minerals provide sound insulation by adding weight to composites. In this study, 25% (w) dense filler and 75% (w) acrylonitrile butadiene styrene (ABS) composites were prepared using a laboratory-type twin-screw extruder. Dense filler powders and ABS/dense filler composites were characterized using a particle size analyzer, a color spectrophotometer, a gas pycnometer, an X-ray diffractometer, a Fourier transform infrared spectrometer, an impedance tube, and techniques such as the Brunauer Emmett Teller method and high-resolution microtomography (mu-CT). The sound insulation of ABS blend with these two minerals as fillers was compared. Magnetite had better sound insulation performance although both fillers were incorporated into the polymers in equal amounts by weight. The better sound insulation performance of magnetite was attributed to its good dispersion in the polymer matrix. The dispersion of the filler particles within the composites was displayed in three dimensions utilizing X-ray mu-CT. Structural analysis was performed on the tomography images, and numerical values expressing the dispersion quality of the filler particles in the composites were obtained.