This item is the third verse, but there are four verses to a cult hymn, each on different music sheets. They are dropped by different members of the Cult. See Music Sheet for an overview.
Be careful! If you "use" the music sheet it will disappear and you will see a message in red: You have learned the third part of a hymn. If you already know that verse, you can not use it again, when attempting to do so a message will appear You already know the third verse of the hymn.
You must learn all the verses of the hymn to gain access to the deeper parts of the cultists cave on Vandura (beyond the piano).
By learning all the parts of the hymn, you will earn the achievement Chorister.