They have to be activated properly to take effect and modify the right sound. Weapon_Pistol.Single is when the pistol fires a shot, while Weapon_Pistol.Reload is when you reload it. The soundscripts' activation condition depends on which one it is e.g. Most add-on weapons don't have special reload sounds played in third person, but one can use the “find” reload to play a new one if there is no reload sound. The event which handles this best is animation_event, with “find” attack primary. Note that some weapon bases, like M9K, don't work in a way that allows PAC to override soundscripts. HL2 stuff is in the gmod wiki but add-on weapons' soundscripts have to be found in game sounds.
You can browse the list of soundscripts by double-clicking, expanding, the game sounds category in the PAC asset browser.
Read more on Soundscripts in the Valve Developer Community: Finding Soundcripts You can modify weapon sounds using Soundscripts, which the Source Engine uses and the PAC sound part allows to use them to replace sounds.
