On her sixth post-Sugarcubes studio album, Björk reminds us that voices -- hers, those of Robert Wyatt, Mike Patton, the London and Icelandic Choirs, and others -- are infinitely malleable and fuckin' weird, dude, that their arsenal of sounds is as rich as the most loaded software. Medulla sounds both ancient and avant-garde, hauntingly beautiful and repulsively fascinating. Bolstered by pliable beatboxing from the Roots' Rahzel, along with subtle production tweaks and brilliant arrangements from Mark Bell, Valgeir Sigurdsson, and Björk herself, Medulla is her most compulsively listenable album -- and her most challenging.