Thanks for the work on this - I was loading rhythmbox, alsa, pavucontrol and grilo-plugins to test - loading only rhythmbox and alsa highlighted some missing deps (and as a bonus, I discovered that loading the alsa extension without doing anything else is enough for sound to work on my laptop).
My premise for the minimum set of deps for rhythmbox is that it should be able to play mp3 and ogg-vorbis files (and never mind the 1,001 error messages).
Loading only rhythmbox and alsa, I can achieve the above by adding libvorbis, gsettings-desktop-schemas and libgnome-keyring as deps - adjusted dep file (and for rhythmbox-gir) uploaded.
As an aside, loading dbus-glib does not seem to fix errors of the form "The name org.gnome.blah-blah was not provided by any .service files" - I guess I need to summon up the energy to look into this further at some point. I'm also wondering, since gtk3 is large anyway, whether we should just bite the bullet and load dconf, gnome-icon-theme-symbolic, gsettings-desktop-schemas, etc as deps of gtk3.