From what I understand the entire metadata set of Roon is stored in the cloud. It’s a “custom” dataset built from multiple sources (we know of rovi/allmusic and musicbrainz, Roonlabs may use other sources as well). For all the albums in your Roon library you’ll have a copy of the respective metadata cached locally. You cannot directly access this stuff on a database level but of course you can
- view it in Roon
, - export some of the metadata to Excel or
- access it via the Roon API.
I think for what you want - comparing your library to musicbrainz or discogs ALL - you could try an Excel export of your albums and artists and use this to somehow … … if the metadata was primarily sourced from musicbrainz you will even get a musicbrainz ID so you could match this directly against the musicbrainz dataset. I have no idea if that would be possible for allmusic as well, I believe they don’t have a public API but you get those IDs too.
On the other hand: Roon Radio is supposed to show you what you’re missing in your collection. If you have access to one - or both - of the streaming services supported by Roon it will be a window to the music world you might enjoy much more than comparing data sets.
Also Roonlabs mentioned plans to expand the Discover feature based on what was done for Roon Radio so in the future this could be another way to find out what your collection may lack.