For that we'd have to turn to our good non-core friend, the Psion (Kineticist). He's got a series of powers (Energy Missile, Energy Cone, Energy Ball, Energy Current) which let him choose which type of energy fries the opponent. Every one of these powers has the energies described as follows:

Quote Originally Posted by SRD
Cold
A missile of this energy type deals +1 point of damage per die. The saving throw to reduce damage from a cold missile is a Fortitude save instead of a Reflex save.

Electricity
Manifesting a missile of this energy type provides a +2 bonus to the save DC and a +2 bonus on manifester level checks for the purpose of overcoming power resistance.

Fire
A missile of this energy type deals +1 point of damage per die.

Sonic
A missile of this energy type deals -1 point of damage per die and ignores an object’s hardness.
You'll note, of course, that fire damage normally doesn't explicitly provide +1 damage per die, nor do electricity spells have any effect on saves or SR checks. Therefore, because this power feels the need to explicitly state that a Sonic Energy Missile ignores hardness, I would rule that sonic damage does not ordinarily ignore hardness. Since it's described using the same terms as acid with regards to damaging objects, acid damage is subject to hardness, though it's still good against objects since it's damage isn't divided first.