I've been hoping for such a feature since Lost City HM - the only spell that I'm really sensitive to is Holy. So I'm not playing WHM myself and have the effects of other players turned off. There are mobs in that dungeon who are casting Holy and each time I had to run it, I'd close my eyes during those passages, waiting for someone to tell me that they're dead (or just waiting for battle sounds to stop).

Being able to turn off or at least down would be really helpful.