For any UK users, the AMAZON.MusicCreativeWorkType, AMAZON.MusicGroup and AMAZON.MusicAlbum do not work, so you’ll have to specify your artists, albums and tracks manually in the Interaction Model using Custom Slot Types. Using the plex2csv plugin you can generate a list of all of them easily enough.

The relevant portion of the Intent Schema will then need to change to refer to your Custom Slot Types.

It does mean that you’ll need to update the custom slot values every time your library changes.