Arcane Spell Failure wrote:
Armor interferes with the gestures that a spellcaster must make to cast an arcane spell that has a somatic component. Arcane spellcasters face the possibility of arcane spell failure if they’re wearing armor. Bards can wear light armor and use shields without incurring any arcane spell failure chance for their bard spells.
Casting an Arcane Spell in Armor: A character who casts an arcane spell while wearing armor must usually make an arcane spell failure check. The number in the Arcane Spell Failure Chance column on Table: Armor and Shields is the percentage chance that the spell fails and is ruined. If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.
Barding says nothing about changing this, so it would seem that a creature casting spells or using spell-like abilities will not have to check for Arcane Spell Failure if those spells or actions lack somatic components.
If a creature does somehow have such an arcane power, and its description lists a movement that would quite similar to a somatic component to generate it, like it says they wave their arms around or make an arcane gesture, a GM can rule that they require a check. Not all creatures are written with the idea that they'll be wearing armor, so the designers may not have considered it. I can't think of one of the top of my head, but generally it's not going to be an issue.
