Each sound in SFX/weapons folder has it's autoload plugin stored in scripts/custom/autoload, what you need to do is :
1. Download Bridge Commander Universal Tool (BCUT)
2. Install and then start it
3. Go to BC Tools -> Sound plugins
4. Add sounds, name them and generate plugin.
new plugin should appear in autoload folder, but only if BCUT recognised your Bridge Commander folder
then you simply put the name you've chosen for desired sound into the weapon sound field in hardpoint.