Look at a paca skull and the cheekbones are wrong. Each zygomatic arch is expanded into a broad hollow plate with an opening into the mouth, forming a resonating chamber against the side of the face. The structure is unique among mammals in that form, and it is more developed in males.
Pacas produce a deep rumble or growl, and the chamber is understood to act as a resonator that amplifies and shapes it, with the animal reportedly grinding its teeth against the chamber as well. The call carries in dense understorey where sightlines are short. The chamber's association with a heavily built jaw and large masseter also serves feeding on hard fruits and seeds.
The function is inferred more than measured. Direct acoustic experiments on live pacas are scarce, and an alternative reading is that the enlarged arches are primarily a consequence of jaw musculature with sound production a secondary use. Both readings are compatible with the anatomy as described.