This study investigates the microstructure and acoustical performance of eco-friendly epoxy–wood composites and plant fiber based materials for potential application as sound absorbing solutions. Natural fibers and wood-based composites are being increasingly used rather than synthetic fibers. This is due to their advantageous specific characteristics. They are low in cost (because they ofen are waste in production processes), environmentally friendly, non-toxic, renewable, and biodegradable, as well as abundant in supply. For research purposes, epoxy composites were manufactured with varying oak chips using controlled proportions of resin filler and their internal structure was characterised by industrial computed tomography (CT). The sound absorption coefficient (SAC) was determined using an impedance tube in the frequency range of 100–5700 Hz. The results showed that the acoustic properties strongly depend on the concentration of filler and microstructural uniformity. The highest and most stable absorption values, with coefficients approaching α ≈ 1.0 in the mid- and high-frequency ranges, were obtained for samples containing 65–75% wood chips. With a higher filler content, one can notice an increased heterogeneity of the structure, which directly affects the worsening of the reduction of sound absorption. Results confirm that the SAC coefficient is strongly dependent on the effective porosity and pore uniformity, and that the technological process of filling is of significant importance. Furthermore, the findings confirm that both epoxy–wood composites and plant fiber based materials demonstrate great potential as reproducible, wideband sound and eco-friendly absorbers.