Given that everything in Reim appears to be undead, using a sonic runestaff appears to be an extremely Bad Idea, since, well.

No flares.

To double-check, I went down and smacked around some lords in Varunar, and yep. No flares, there, either. (And just to make sure it wasn't a Sonic Weapon issue, I went back and blew up some more stone trolls & giants, and they got blown away by Air flares just fine.)

.

I'm curious, though: does the sonic weapon drop to only +10 when being calculated for DS while facing undead, also? (Because my defenses seemed a little lackluster in Reim.) Or do you get full defensive benefit regardless, and I was just imagining things?