Vegetal wools are a bio-based insulator which building applications are limited due to their flammability. To overcome this problem while respecting their bio-based nature, it seems relevant to identify and apply an environmentally friendly fireproof treatment. However, the impact of this treatment on the acoustic performance of the vegetal wools is not examined in the literature. The objective of this work is to evaluate the efficacy of two phosphorus-based fireproof treatment methods i.e., impregnation and pulverization, and their impact on the acoustic properties of hemp fiber stacks. To assess the efficacy of the fireproof treatment, the amount of phosphorus grafted and the thermal degradation of the hemp fibers were evaluated through inductive coupled plasma (ICP) and thermogravimetric analysis (TGA), respectively. On the acoustic side, the impedance tube test results showed that the treatment can negatively impact the acoustic performance of hemp fiber samples, by the reduction of the sound absorption coefficient. To better understand the effects of treatments on the porous structure characteristics of the samples, parameters such as porosity, resistivity, tortuosity, etc. are determined and the acoustic absorption of the stacks is simulated using the JCAL modelling method.