Acid traps have been bugged for a while now... I believe it had something to do with an update from Bioware, which they address but likely didn't get changed with our lewt script... Blame moth.
Sonic damage is a will save... *Cough cough*
One thing a lot of people don't know is that only 4 traps in the game are effected by Evasion, being Fire, Electric, Acid Splash (not Acid Blob), and Spike. Additionally, you do make reflex savesfor Acid Blob and Tangle, but for Acid Blog, you are only avoiding the paralysis effect, and for Tangle, you are avoiding getting... tangled.