HA ! "Mystery" solved!
Congratulations, you are the first one to report what turns out to be a code error that goes back
at least 17 years
If a weapon with that warhead had a HE kill >13 it was given a different sound..... but the wrong one for a MG
All fixed now
EDIT
This is so funny it needs to be told
This appears to be an error going back to the very beginning and there are DOZENS of weapons that fit the Warhead size 1 / HEkill >13 criteria but I checked and every single one that fits that criteria has a sound assigned in unitweapons.......
except the French one you found
I'm starting to think because these all have assigned sounds and that is rare in most cases that it was noticed early...."fixed" using assigned sounds in unitweapons then cut and pasted into various OOB's so the fix was forgotten about and it appears the only ones that slipped through was the one in the French and Vichy OOBs
This is not just one or two units that fit the criteria. Anything 13mm does as does any 50 cal twins or quads ..about 50 units in total