to put it basically, the file in \Autoload is what links the sound file with the torpedo file.
it goes from the file, to the "alias" (sound pack file), to the projectile to the hardpoint.
so, if you name you sound "aehlla_torp.wav", then you say in the autoload file
Foundation.SoundDef("sfx/Weapons/aehlla_torp.wav", "AehllaTorp", 1.0)
then in the hardpoint you call Tactical.Projectiles.AdonisTorpedo (for example) and then in the torpedo file you call "AehllaTorp", not "aehlla_torp". you call the "alias" of the file.
if you're sure everything is linked propperly, then the sound file is corrupt. but first be sure you don't have a typo around there.