Any arcane spellcaster who casts a spell requiring a Somatic component, suffers a chance of spell failure if s/he is wearign armor at the time.
Bards, of course, tend to pick spells with Material and/or Verbal components only, so can often cast many of their spells without worrying about Arcane Spell Failure %'s.
However, even bards, when casting spells, do more than SING. Their Somatic components are often simply the physical manipulation of their instruments (specific, complex fingerings on the holes of their flute, or chords on their harp, lute, guitar, etc, and so on ...). However, bulky, restrictive armor can still interfere with such fine, precision movements ... so arcane spell failure chances still apply.
IOW ... as with ANY Arcane spell, if armor is worn, the Arcane Spell Failure % applies to all spells cast which have a Somatic component ... wether the caster is a BArd, a Sorceror, a Wizard, or anything else for that matter.