Muzzlebrakes are not regulated in the united states - they do not reduce the sound of the rifle, but s ilenc ers/noise reducers/ that can be attached to a firearm (powder burner) are VERY much so, along with automatic weapons. These both require special permits from the Federal Gov't ATF bureau and background check by the FBI and usually a couple thousand $ fee, along with a nominal renewal fee and being subject to periodic re-checks for criminal activity by FBI.
Airguns are not regulated in the United States, though the more powerful they become, we may unfortunately see a change in this policy. If a noise reducing device like that on the Whisper model air rifle is
built into the product and not designed to be removable then it technically falls outside the purview of the Federal Gov't. If one were to try to build or modify such a device for fitment or potential attachment
to a firearm, that is another story altogether and would be highly illegal, unless one was already licensed to do so by the Government. Heavy fines and felony indictments await those who would ignore these laws and got caught.