People who bought the Apex expansion have the new realistic thunder (lightning) sound.
My friend who doesn't have Apex still has the old thunder samples which sound like a tinfoil tray being hit with a wooden spoon.

BUT:
Playing on the old maps (Altis & Stratis) with Apex expansion plays both the old and new thunder samples simultaniously.
Playing on the Tanoa map plays just the new thunder samples which sound amazing by the way :-)

The old thunder samples should be removed from the game completely and replaced with the new realistic ones, so that everyone has the same sounds ingame :-)

Cheers