A word for authorship of song’s music, but not lyrics? [closed]

You can use composer to indicate such authorship.

A composer (Latin compōnō; literally "one who puts together") is a musician who is an author of music in any form, including vocal music (for a singer or choir), instrumental music, electronic music, and music which combines multiple forms.
_Wikipedia

The word seems to be commonly used in the sense that you mean. Take for example this article about John Lennon and Paul McCartney:

Unlike many songwriting partnerships that comprise separate lyricist and composer, both Lennon and McCartney wrote words and music. Sometimes, especially early on, they would collaborate extensively when writing songs, working "eyeball to eyeball" as Lennon put it.
_Wikipedia

I'd suggest composition for what you describe

i.e. http://www.example.com/songs/composition/blah.html

That would suggest that the contents are musical creations rather than (as has been suggested) web pages about different composers. Although I wonder if this kind of taxonomy is really necessary?

Settings has something of the meaning you want, though I am not sure how clear it would be to the visitor. "Settings of other people's words" would be clearer if longwinded.

A musical setting is a musical composition that is written on the basis of a literary work. The literary work is said to be set, or adapted, to music. Musical settings include choral music and other vocal music. A musical setting is made to particular words, such as poems. By contrast, a musical arrangement is a musical reconceptualization of a previously composed work, rather than a brand new piece of music. An arrangement often refers to a change in medium or style and can be instrumental, not necessarily vocal music.

Wikipedia