I am a bit confused. I did a test and placed empty sound for one unit in data (PLCHSHOT.82m). Then added the code:

!?BG0;
!!BG:A?v4; action shoot
!!VRz10:S^..\data\newshoot.82m^;
!!SN&v4=7:Pz10; Play it now !

When Powerlich shoots I hear the new sound. When other units shoot I hear only the new sound too (normal) but I should hear both sounds, the original and the new one, as I did not placed their respective empty shoot snd in data.

So far, it seems useless to place an empty attack sound in data. The new 82m replace the original even with different name. Maybe thats how it should work, I was not sure. The good thing is that we can play a different custom sound for each action (defend, move, shoot, wait) but surely you all already know this. The only problem I have is for !!BG:A6; (walk and attack) as I added walk sound compiled with attack sound but the attack sound comes either too early or too late, regardless how far the unit moves...

I will try to remove walk sound from A6, maybe it should have only attack, despite the ERM name.