soundTypeIndex is used in muzzle devices class ItemInfo to select a weapon soundset to be played when said muzzle device is attached to the weapon.
This how for example suppressors get suppressed sounds when weapon is fired.

However, there is no way to make subsonic ammo sound different from regular ammo unless you resort to very convoluted scripted stuff with fake muzzle device, which then gets more convoluted ad nauseam the more different muzzle devices a weapon supports, etc.

So, would it be possible to have at least magazine class support soundTypeIndex?