Currently hand grenades and underbarrel grenade launchers (incl. static and mountend) configurations use the same sounds

But look at this, the config and game data already mention separate sounds for the grenade launcher explosions, they are not used in any.
If I'm right, then just was forgotten to change the sound set names in base class CfgAmmo → G_40mm_HE → SoundSetExplosion[]={};
It turns out that the configurations of these two types of weapons need to be make looks like (here is text version - https://pastebin.com/h1KAbNHW) :

After testing could not able hear any technical reasons why these sound sets were not used