In August 2004, Björk released Medúlla. During production, Björk decided the album would work best as an entirely vocal-based album. This initial plan was modified, as the majority of the sounds on the album are indeed created by vocalists but several feature prominent basic electronic programming, as well as the occasional musical instrument. Björk used the vocal skills of throat singer Tanya Tagaq, hip hop beatboxer Rahzel, Japanese beatboxer Dokaka, avant-rocker Mike Patton, Soft Machine drummer/singer Robert Wyatt, and several choirs. She again appropriated text from E. E. Cummings for the song "Sonnets/Unrealities XI". At the time, Medúlla became her highest charting album in the US, debuting at number 14.
In August 2004, Björk performed the song "Oceania" at the Opening Ceremony of the 2004 Summer Olympics in Athens. As she sang, her dress slowly unfurled to reveal a 10,000 square foot (900 m) map of the world, which she let flow over all of the Olympic athletes. The song "Oceania" was written especially for the occasion and features the talents of Shlomo, a Leeds-based beatboxer, and a London choir. An alternative version of the song began circulating on the Internet with additional vocals by Kelis. It originally appeared on the promotional "Oceania" single released to radio stations and later became available to the public as a B-side of the "Who Is It" single, which charted at number 26 in the UK. This was followed in early 2005 by "Triumph of a Heart", charting at number 31. A video for the potential next single, "Where Is the Line", was filmed in collaboration with the Icelandic artist Gabríela Friðriksdóttir in late 2004. This was initially a sequence from an art installation movie of the artists but was released exclusively on the Medúlla Videos DVD as an official promo for the track.
Björk's 2004 studio album, Medúlla, is almost entirely constructed with human vocals. musicOMH's review stated that: "Despite its voice-only premise, Medúlla shows off a mile-wide scope of influences"; noting elements of folk and medieval music. Wondering Sound wrote that despite "its comparative starkness, [Medúlla is] every bit as sensual as [Vespertine]." The publication also added: "The electronic treatments range from industrial distortion to percussive glitches and dreamy layering, rarely descending into novelty." The album combines beatboxing, classical choirs that suggest composers like Penderecki or Arvo Pärt, and "mews, moans, counterpoint and guttural grunts" provided by Björk and guests like Mike Patton, Robert Wyatt and Tanya Tagaq. Medúlla includes "vocal fantasias" that lean toward chamber music, alongside tracks that "are obviously but distantly connected to hip-hop." Glimpses of Bulgarian women's choirs, the polyphony of central African pygmies, and the "primal vocalisms" of Meredith Monk were also noted.
After the tsunami which struck Southeast Asia in late 2004, Björk began work on a new project titled Army of Me: Remixes and Covers to help raise money for a relief fund. This project recruited fans and musicians from around the world to either cover or remix the 1995 track, "Army of Me". From over 600 responses Björk and her co-writer Graham Massey picked the best twenty to appear on the album. The album was released in April in the UK and in late May 2005 in the US. By January 2006, the album had raised around £250,000 to help UNICEF's work in the southeast Asian region. Björk visited Banda Aceh in February 2006 to view some of UNICEF's work with the children who were affected by the tsunami.
Björk has also taken an interest in environmental issues in her native country. In 2004, she took part in the "Hætta" concert in Reykjavík, organised in protest against the building of Alcoa aluminium smelters in the country, which would make Iceland the biggest smelter in Europe. She founded the organisation "Náttúra", which aims to promote Icelandic nature and grassroots industries. In October 2008, Björk wrote an article for the Times about the Icelandic economy and her thoughts on the proposed use of natural resources to get the country out of debt. Björk, in collaboration with Audur Capital, set up a venture capital fund titled "BJÖRK" to support the creation of sustainable industries in Iceland. She has written the foreword to the English translation of the Iceland bestseller by Andri Snær Magnason titled "Dreamland".
In 2004, Leila discovered the work of Finnish multimedia artist Heidi Kilpeläinen, who had taken her combination of lo-fi, homemade electro pop with her own self-produced music videos and combined them under the alter ego character, HK119. Leila soon referred HK119's work to Björk, who started mentioning HK119 in various press and interviews. In 2004, Björk announced HK119 as her favourite act of 2004 and cited her as "The Perfect Blonde Woman". HK119 was soon signed to Björk's parent label One Little Indian Records, which released her debut album in 2006. HK119 and Björk appeared in a joint interview in Dazed & Confused magazine in 2006, where Björk stated about HK119's work, "It's unique. Even if I gave you $3 million, you couldn't improve on it... [Its] simplicity is [its] strength." HK119 later released her albums, Fast, Cheap and Out of Control in 2008 and Imaginature in 2013, both on One Little Indian Records.