Virtual Soundtrack

The distant cousin of the Song Fic, this is the practice of annotating a Fanfic with music cues in such a way that, if followed, produces the rough equivalent of a motion picture's soundtrack for that fic. Very often these are the same pieces of music that inspired the author as he or she was writing. Regardless, they are intended to provide extra atmosphere and drama to the written material.

The cues themselves can range from unobtrusive citations styled like pullquotes and set off from the story text proper, to elaborate instructions as to where to start playing on the track and for how long, often delivered by the narrative voice as part of the story. More recently, embedded links to YouTube videos have started to replace more intrusive text cues.

Some authors may provide soundtrack listings in separate files, but these qualify more as supplemental material than examples of this trope.

One key element which defines a Virtual Soundtrack is that it is optional. You can ignore a Virtual Soundtrack cue without losing anything in the story, unlike the Song Fic, where you must wade through the lyrics to extract the story. Also, if the music is part of the action—for instance, in the description of a concert—it does qualify as this trope. (Although a listing of all the pieces played at the concert would be a good candidate for supplemental material.)

This trope is usually limited to larger pieces of Fan Fiction, on the order of a novellette or longer.

A popular variation on this trope is creating "Fanmixes", fan created albums with songs the fans think their relate to a certain character or ship, but not necessarily tied to a fanfic. Usually compiled by fanauthors, and the production quality ranges from playlists posted in the fan's blog in LiveJournal or Tumblr, to actual zipped compilations with album covers and PDF booklets. Thanks to the rise of YouTube and on-line music services that let you share playlists like Spotify, creating this sort of content is easier than ever.

Examples of Virtual Soundtrack include:

The "pullquote" version is a common feature in stories produced by Eyrie Productions, Unlimited, although it is by no means universal. It is particularly prevalent in the Future Imperfect era of Undocumented Features, especially works written mainly by Gryphon.
Chapter five of Eric Hallstrom's epic Ranma and Akane: A Love Story begins with the explicit note, "By popular demand, the majority of this episode should be read to Fortuna Imperatrix Mundi from Carl Orff's Carmina Burana" and offers a (long-defunct) link to an MP3 of the piece. When the great battle in Hell begins, the narration pauses to specifically inform the reader to start playing it, and much later bookends the action with the note "You can turn it off now."
- RAALS verges on this trope elsewhere, as well, with a large number of songs, often from British/Irish/Scottish folk music tradition, that are sung throughout the story by the various characters. MP3 links were provided for these as well. This only edges up to the trope because the song lyrics are provided whole within the story, and are part of the story rather than supplemental atmosphere. In this regard, RAALS is much like a musical rendered in text.
Bubblegum Disaster by dialNforNinja includes soundtracked sections.
The parody Lemon Mega Crossover Lemon Sherbet by John Biles.
Genma's Daughter and its sequel Notes from Julliard, both by Deborah Goldsmith, drift into this trope with in-story violin performances of specific classical pieces that a dedicated reader can locate easily enough; in addition, Goldsmith provides recommendations for (and sometimes links to) specific performances which inspired her work.
In the Warrior Cats trollfic Starkit's Prophecy, in the final chapter, the author's note recommends that the reader listen to Romeo and Juliet Overture by Tchaikovsky.
The Homestuck fanadventure Promstuck occasionally links to music to depict what is playing in-canon at the time.
Higher Flier, or My Little Blackbird by Admiral-Tigerclaw.
Surprisingly, this is not limited to fanfic! Foreshadows: The Ghosts of Zero is an eBook from Baen which includes an archive holding two hours of original music to which the text of the eBook links. The blurb about the book at Baen's website explicitly calls it an "illustrated anthology and soundtrack album in one."
As of season 3, the players of The Massive Multi Fandom RPG sometimes include YouTube links to appropriate music snippets for whatever they are doing at the moment.
The My Little Pony: Friendship Is Magic fan-comic A Brony's Blue advises listening to the author's chosen songs to "view the story to its fullest quality".
The Halo fanfiction The Life usually mentions music being played on one of the squad member's loudspeakers.
The Homestuck fanfic Cities in Dust (which was also created by the author of Promstuck) has, interspersed in the text, links that direct to YouTube videos of songs that should be played as the background soundtrack of the scene. When the author finally concluded the fic, she posted a list of what songs were supposed to be in each episode, which helped to find the songs whose original YouTube link disappeared because of copyright reasons.
The Persona 4/Jo Jo's Bizarre Adventure crossover fic A Different Kind of Truth has a version posted in the Spacebattles.com forums where, along with the snippets of WIP and author notes, he posts a version with links to what song should be playing in certain scenes (usually themes from either the P4 or the JJBA soundtracks), along with a playlist of whose songs inspired the author writing each episode. In fact that's a signature of said author: the Spacebattle version of his fics tend to have links to the appropriate soundtrack for the scene.
Undertale fanworks definitely love this, specially given the abundance of remixes of the admittedly awesome soundtrack. Many AU fancomics (like Storyshift and Inverted Fate, to just name a couple) have even music specially created for them.