Audacious 3.6 tarball contains an appdata file in "contrib" subdir.
It doesn't validate yet, however.

xmp-plugin-audacious rewritten for Audacious 3.6-alpha1 C++ API
https://github.com/mschwendt/xmp-plugin-audacious
XMP upstream has been made aware of the major changes

Audacious 3.6 introduces C++11 usage and bumps the plugin API to 46

xmp-plugin-audacious ported to Audacious 3.5 API
https://github.com/cmatsuoka/xmp-plugin-audacious/pull/2

audacious-plugin-fc ported to Audacious 3.5 API in upstream cvs

TODO (examine): amidi plugin drops the ALSA backend (intentionally)
  but also the amidi-plug directory - is the fluidsynth backend built-in?

Audacious 3.5-devel bumps API version to 45 (also minimum)

Input plugin API to change in 3.5:
http://audacious-media-player.org/news/22-input-plugin-api-to-change-in-3-5