Silence

From NWNWiki

This article is about the effect; for the spell, see Silence (spell).

Creatures under the effects of silence make no noise while moving but are also unable to hear anything themselves. Spellcasters will be unable to cast spells with V (verbal) components if silenced.

Bards are unable to start a bard song or curse song while silenced, but silence does not nullify existing effects of these songs. A silenced creature cannot benefit from a new bard song, but can be subject to a new curse song (which is likely a bug).

One common source of this effect is the silence spell.