I have no horse in this race, and I have not witnessed this happening, but I am certainly familiar with ear plugs, and just reading this thread has brought up one big thought in my mind: even with modern day technology, it's difficult to get good earplugs that completely deafen. I've tried using ear plugs at multiple times in my life, and never once have I encountered a pair that completely canceled out loud noise, it just made it bearable. Most ear plugs are designed to muffle enough for hearing protection, but if someone's doing a magically powered scream, something that has effects based on more than just how loud or shrill it is, it really doesn't make sense for mundane earplugs- especially those made with medieval technology- to be able to completely block that out at all ranges.
I'd personally agree with the idea that if you want something to prevent magic from happening, it needs to be an item in your inventory, needs to be specially made, and needs to carry a drawback with it. If it's strong enough to block magic, it's strong enough to prevent any communication that's not by sight. If you're talking to each other, they aren't strong enough. And at bare minimum, you need to do a prefix or something to indicate that you've put them in so you can't retroactively powergame them.