It's hard to accuse Björk of making music influenced by commercial or critical expectations at any point in her career, but her post-Homogenic work is even more focused on following her bliss, whether that means acting and singing in Lars Von Trier's grim musical Dancer in the Dark; crafting tiptoeing laptop lullabies on Vespertine; or, in the case of Medúlla, sculpting an album out of almost nothing but singing and vocal samples. The album's title and concept refer to the purest essence of something, and Medúlla explores both the ritual power of the human voice and some of the most essential themes of Björk's music in a way that's both primal and elaborate. It took a large cast of characters to make the album's seemingly organic sound, including vocalists ranging from Icelandic and British choirs to Inuit singers to Mike Patton and Robert Wyatt…