Inverse Frequency Sound format
The Inverse Frequency Sound format was used for sounds in some Apogee, id, and Softdisk games. It is closely related to some other formats including Crystal Caves Sound format and a format used for some of the sounds in Wolfenstein 3-D.
There is a 16-byte header (of which only 10 bytes are actually used, the rest being nulls), and a sound definition table preceding the section with actual sound data.
Games using this format included Catacomb, Commander Keen 1-3, Cosmo's Cosmic Adventures, Dangerous Dave, Dangerous Dave in Copyright Infringement, Dangerous Dave 2, Duke Nukem, Duke Nukem II, Hovertank 3-D, Math Rescue, Major Stryker, Monster Bash, Paragon, Pickle Wars, Realms of Chaos, Rescue Rover, Shadow Knights, Slordax, Word Rescue, and Xenopods.
There is no fixed file extension for this because the file is usually named sounds.xxx where xxx is replaced by a particular extension for the specific game it's used in (e.g., hov for Hovertank 3-D), often shared with various other game files in different formats.