Hi Petter and Martin, mpd maintainer here, we're using libvorbisidec on armel as an alternative to vorbis, however we may have to disable it for buster as the upstream build system moved to meson and needs the pkgconfig file to detect vorbisidec. This shouldn't be too hard to fix, don't you think?
Also, libvorbisidec has an RC bug (#899907) filed against it and probably needs a new maintainer address for the upcoming buster release, just like the other 5 pkg-xiph packages. Have you considered creating a tracker.d.o team (see https://qa.pages.debian.net/distro-tracker/usage/teams.html) and use that address? Or do you intend to integrate the packages into the multimedia team? I can lend a hand if you indicate the direction into which you want this to go... Florian