Acid and sonic attacks deal damage to most objects just as they do to creatures; roll damage and apply it normally after a successful hit.
I interpret this to mean that they ignore hardness... they deal damage just as if it was a creature.. creatures don't have hardness. You roll damage and apply it. Thats it.
I can see how it could be interpreted that hardness applies to acid/sonic, the prior was my interpretation of the passage. It is a little bit unclear, the "Hardness" section specifically says that you apply hardness to all damage an object takes.. this could be overridden by the "as if it was a creature" but its very vague. The acid/sonic section should probably say "ignoring any hardness."
A case could be made for either, but my own personal opinion (ignoring the FAQ) is that acid/sonic ignore hardness. Which makes sense in some situations but not others... if you put a lump of iron in some concentrated acid it will dissolve pretty quickly, but glass in acid is usually pretty happy.