I've got a feeling there's at least one Child ballad where a character is under a spell that means he or she cannot speak.

And just for fun, here's a link to a modern pastiche I found looking around which has someone under a spell which is taken as indicating she is dumb. Except she isn't by a long chalk. Written by one Brandi Pastories "for an English Literature class" and not at all bad.'Ballad of the Shepherd Boy'