These are the sound files for TA. Buttons, weapons, explosions you name it these are them.
All sound files need to be placed in the sounds directory, with the exception of mission briefings, which go in the briefs subdirectory of camps. Since these are only used for the missions, most unit makers will never have to deal with them.
Using the sounds is quite straight forward for custom weapons, see the Weapon section for more information on that. Having custom move, select, etc sounds for your unit is a little more complex but not much. Check out GameData for that use.
Wikipedia entry on WAV files: