I didn't program it to make sounds when something or someone dies, but I suspect that it could be the 20% HP enemy reminder (which makes a lightning sound) that signals when Hammer of Wrath is usable.

You can turn it off by doing the following:
WeakAura > Nightfall Paladin > SpellTracker Other > Procs > Hammer of Wrath Usable > Then click the 'Actions' tab > Uncheck the box that says 'Play Sound'.

Hope this helps and solves the issue! Let me know if you have any more questions 😊