[Noxid]

With the fact that sound and acid are pretty damn rare elements, there really shouldn't be any need for an item to give a number as big as other resists get against them, so I don't like those items.

No comment on the feather coat though. Other people can determine whether or not that'd be good.

Dynamic sound tomes are far from rare these days and you also have rune mage's elemental rune which takes the damage type of your tome as it's element. I'd agree acid isn't really common but you see/hear sound damage plenty enough now to warrant some form of resist item.