You'll have to ask the Yale music crowd why it isn't on Hackage.... But, their web site does have a page on how to get it: http://haskell.cs.yale.edu/euterpea/download/  But it basically boils down to getting it from github: https://github.com/Euterpea/Euterpea - If I recall, it has one or two dependencies that didn't build on the latest HP, but just required tweaking of their .cabal files to fix.... Wish I had written that down when I started!​