I have an api at
https://developer.musicindex.org and a basic search at
https://musicindex.org.
I'm also playing around with a tool for embedding RSS metadata directly to the file... So far it works for mp3 id3v2.4 tags, but I still have to map out the fields to support FLAC/ogg and MP4/MOV files.
As for the fields themselves, I did try to take as much as I could from the Musicbrainz playbook, but we still have no real definitive database of Artists, People, Releases (recordings), Release_Type (EP, Album, single) or Works (actual song entity linked to releases/recordings/covers/parody of said song)...
The Musicbrainz database is very thorough for creating meaningful and explicit relationships between fields, but so far in the RSS namespace we still have very rudimentary fields that can just barely determine Artist, Track no., Title, release_date... And the rest is up to interpretation: eg, feed itunes:author = albumartist, item itunes:author = track artist....
My conclusion is that it might be worth developing tools that submit data to Musicbrainz directly. Then, a MBID={guid} could be used in RSS to represent everything cleanly to any Musicbrainz-aware app