MusicBrainz scope isn't easily discoverable, and it comes up in edits, so it doesn't seem obvious to everyone.  Users often ask for guidelines from the people that vote against their edits, so it'd be great to point to something official rather than regurgitate standard operating procedure (as in any game of telephone, regurgitating information runs the risk of misunderstandings).  Several excerpts from the existing documentation:

1. https://musicbrainz.org/doc/About says "the scope of the project [is a] music encyclopedia".  Likewise, https://metabrainz.org/projects says MusicBrainz is a a"music encyclopedia that collects music metadata"
2. https://musicbrainz.org/doc/Beginners_Guide#Definitions defines a release as "a specific issuing of an album, single, compilation, etc., and includes a specific set of recordings in a particular order"
3. https://musicbrainz.org/doc/Beginners_Guide#Adding_a_release says that "home-made" releases that are "not widely available" nor useful beyond solely "the individual who created them" are removed from the database.

The fact that https://metabrainz.org/projects shows that MusicBrainz and BookBrainz are separate projects ought to suggest this, but I'd recommend explicitly writing these bullet points:

• A book narrated on an audio medium (an audiobook) is within the scope of MusicBrainz, and can be represented as both a Release and a Work (can it?), but the original book itself would not be a MusicBrainz Release.  (Can a book be represented as a MusicBrainz Work? I don't see a work type for that.)
• A movie soundtrack is within the scope of MusicBrainz, and can be represented as both a Release and a Work, but the movie itself would not be a MusicBrainz Release.  (Conceptually, the soundtrack aspects of the movie itself would be represented as the MusicBrainz Work.)
• A video game soundtrack (or an unofficial gamerip) is within the scope of MusicBrainz, and can be represented as both a Release and a Work, but the video game itself would not be a MusicBrainz Release, unless the video game itself is something like a Mixed Mode CD.  (Conceptually, the soundtrack aspects of the video game itself would be represented as the MusicBrainz Work.)

Additionally, I would document what seems to be the endorsed practice for entering enclosures:

• If a music or audio disc is enclosed/bundled within a non-audio product such as a movie or game, the audio disc should be entered as a Medium of a Release, but the movie/game is ignored.  However, If the movie disc contained dedicated tracks for things like music videos or concert footage, then it should also be entered as a Medium of the Release.

I'd also re-state from the other documentation, as these cases have also come up in edits more than once,

• MusicBrainz tracks real music that has been or will be made available, including unauthorized music such as remixes.  "Fan-fiction" music that doesn't exist should not be entered into the database.