Went over this with a few people yesterday, in fact...
Uller: You are partly correct. Undead are immune to things that require a fortitude save unless the thing also effects objects.
So, on that sonic attack - imagine this - if it's the sort of thing that does damage by "heavy stun" or shocking internal organs, then the undead don't care. If it's the sort fo sound that cracks stone, the undead are just as vulnerable as everyone else....
Now, on whether you can Haste undead - by the strictest reading I can find, the answer is "only if he wants to be hasted". No errata is needed.
Check out the PHB, pg 150 "Voluntarily Giving Up a Saving Throw". A creature can choose to forego a saving throw, and willingly accept the spell's result. Note, that's "forego", not "automatically fail". If the creature wishes, the save doesn't happen. If it doesn't happen, it certainly isn't required. This specifically can get through immunities - the text refers to an elf setting aside his immunity to sleep effects as an example.
So, if the undead has choice, it can set aside the save - you can haste intelligent undead, if you tell them what you plan first. Liches can use Polymorph Self, and so on.