Soundproofing component, in particular for a firewall of a motor-vehicle passenger compartment

The component (10) has a part (18) comprising an opening (20), and another part (22) connected to the former part in the opening. A fixing unit (27) fixes the latter part on the former part and includes complementary fitting forms formed of slots and teeth placed over the parts to allow fitting of the latter part with the former part. The former part has a peripheral reception zone (24) in a periphery of the opening to receive the latter part. The latter part is delimited by a circumferential zone (26) complementary to the peripheral zone and arranged in contact with the peripheral zone.