Restoration of historical buildings should be carried out without giving damage to the existing historical building, and material selections should be determined in accordance with the historical building structure. In addition to the selection of materials compatible with the historical structure, the selection of materials that can improve the interior acoustic quality of the space is one of the important parameters. In this research, increasing the sound absorption coefficient in the plaster layer was investigated to enhance the acoustic quality of historical places. Accordingly, plaster layers prepared with a natural hydraulic lime binder, which are frequently used in restorations, were examined in this research. It was aimed to increase the sound absorption coefficient by making some content changes in the mortar mixture used in the plaster layer. Within the scope of the research, the effects of the lightweight aggregate type used in the plaster layer on the sound absorption coefficient were investigated. The use of flax fiber and polypropylene fiber additive in the plaster layer produced with perlite aggregate was researched, and its effects on the sound absorption coefficient were detailed. Also, the utilization of crumb rubber instead of perlite aggregate was examined, and its effect on the sound absorption coefficient was investigated. In this study, besides sound absorption coefficient tests, compressive strength, flexural strength, open porosity, and capillary water absorption tests were performed. It was determined that the use of lightweight aggregate type can positively affect the sound absorption coefficient. It was identified that polypropylene fiber additives can increase sound absorption at high frequencies and flax fiber additives can increase sound absorption not only at high frequencies but also at low frequencies. It was observed that the use of crumb rubber additives can significantly increase sound absorption at low frequencies and it can enhance sound absorption at high frequencies. As a result, systematic research was performed to increase the sound absorption coefficient in the plaster layer.