The score/soundtrack for The Host will be released on March 26, 2013, three days before the film is released on March 29, 2013. The score is by Antonio Pinto and will have one non-score track, “Radioactive” by Imagine Dragons.
The soundtrack will be available on iTunes, but I cannot find a link to pre-order it. I am also not finding it listed on Amazon yet.
Update: You can buy a physical copy of the soundtrack on Amazon as an import.
MP3 of the score is now available on Amazon! (There are 27 tracks!)
Source: The Host Facebook Page