This seemed to be bugged. The boss in the Boiling Mud plane quest boss didn't suffer spell fail on any spells (all of which were Verbal component ones) and it seemed some of the ghosts in the Opera house cast just fine too. The sonic immunity did work though
Silence setup is pretty standard nwn on how it works.
If there is hope to check what exactly is going on, we need more information.
Who you casted silence on as target, was the target the creature you speak of and did not roll saves/resists or anything. If you casted it on yourself, and you are simply trusting on effect of the zone, were there saves or anything else etc.
Sorry, yeah. I cast it on myself, and set up a zone. I engaged the monster and trapped it in the zone. It started getting spells off from within the zone and ran away. That said, I think the boss might have been a drow, thus SR, thus I will check this again and report back.