MOD is a MODIFICATION of some parts of a game
Back in the day, making things from scratch... I.e. a duke nukem game turned into a vietnam shooter would be called a Total Conversion = T.C. and a Mod was just editing some of the files... I.E. Making jetpack useable whenever, rpg fires 100x faster... guys ahve more health... sounds were changed... added a few new guns etc. all mods...
'Total conversion' Never hung around though... so people just use 'mod' now... E.G.
Counterstrike is a Mod for half-life
Your soundmod is a Mod for T-72...
Both are modifications... doesn't mean squat in what the difference is... YOU WIN lol