The acoustic propagation characteristics within oil-immersed transformers are complex, which limits the effectiveness of acoustic detection for discharge defects. This study quantitatively investigates the acoustic propagation properties of oil paper insulation and characterizes the variations in acoustic waves generated by partial discharge (PD) during propagation. The results show that the acoustic velocity in insulating oil remains nearly constant across frequencies, with attenuation primarily dominated by diffusion effects. In contrast, Oil-immersed insulating pressboard (OIP) exhibits significant acoustic anisotropy, with velocities along the surface and thickness directions ranging between 3–4 km/s and 1 km/s, respectively, both closely related to pressboard thickness. When OIP is introduced, both the peak amplitude and equivalent frequency of acoustic waves decrease with increasing pressboard layers. Notably, the peak attenuation is more significant for low-energy discharges, while the relationship between equivalent frequency and discharge energy remains unaffected by the pressboard. These findings provide critical parameters and a theoretical foundation for the acoustic detection of PD in power transformers.