A number of reviews I've read have said that Volta is a return to Björk's earlier sound. I can see why, with "Innocence"'s "Big Time Sensuality" style vocals combined with a Homogenic electronic sound; or the punk Sugarcubes-esque "Declare Independence". But, while she has returned to the eclectic use of instruments, I think the album has the same general feel as Medulla. At times it hovers just on the edge of musical chaos while still being vastly more listenable than her previous album (I'm disregarding The Soundtrack to Drawing Restraint 9, which is obviously not a true Björk album as it doesn't have a one-word title :).
There's also copious use of brass. Some tracks are almost entirely brass with vocals ("The Dull Flame of Desire", "Pneumonia") giving a deep resonant quality to the album. There are even ship horns which sound like brass at the end of "Earth Intruders" which, incidentally, has quickly become one of my favorite Björk tracks (apart from the bit with the ship horns).