From Discworld MUD Wiki
Speech is a required power for some rituals.
The following things remove or interfere with it sufficiently to prevent these rituals:
- Being under the effects of the Silence ritual.
- Being underwater and not having gills (gills allow you to speak well enough to perform rituals).
- Being a frog as a result of the spell Stacklady's Morphic Resonator. (Well you can say "Ribbit" but that's not a big help.)