You have 2 options:
Option 1: change it in the weapon txt files, like VTF crosshairs. For example this is SoundData in tf_weapon_grenadelauncher.txt
SoundData {
"single_shot" "Weapon_GrenadeLauncher.Single"
"reload" "Weapon_GrenadeLauncher.WorldReload"
"special3" "Weapon_GrenadeLauncher.ModeSwitch"
"burst" "Weapon_GrenadeLauncher.SingleCrit"}
Option 2: changing game_sounds_weapons.txt. I have mine in my VTF crosshair Scripts folder alongside my weapon txt files. Here's a download for a version i have on dropbox. I'm uncertain which changes I made. My guess is I lowered scattergun sound volumes
You can also use game_sounds_weapons as a reference for cool sounds to replace others with. So it's nice to have even if you use weapon txt files
For example you can find AWPer HAND as "CSGO AWP" in game_sounds_weapons, change its volume, or copy the wav and make the stock sniper play the same sound. Or make your ubersaw sound like an AWP. World is your oyster
A quirk of the source engine is sometimes it might still play the stock sound. It happens less if you post multiple lines of the wav in the section. The original with stock sounds has the issue. The file that gets shared to change that one copies the line stock sound around 300 times in the original section which makes it work 95% of the time.
I've no clue if the AWP has the issue
I like the loch n load dud for a quiet scout pistol (only changes your own pistol's sounds)