media-plugins/audacious-plugins[ffmpeg] depends on virtual/ffmpeg, but it silently fails at configure time with media-vide/libav, while with media-video/ffmpeg it works just fine. The result is that the ffaudio plugin is not compiled. The cause seems to be that audacious-plugins depends on version information of various libav* libraries, which is not compatible between libav and ffmpeg, with libav having generally lower version numbers. The version checks are present not only in the build system, but in the actual code as well, to choose between code paths. After hacking the build system to accept libav, it seems to actually work just as good as with ffmpeg.
I'm attaching a patch to configure.ac to take care of reporting an error when we passed --enable-ffaudio but configure thought we don't have ffmpeg available. I already sent a bigger patch upstream: https://github.com/audacious-media-player/audacious-plugins/pull/10
Created attachment 338558 [details, diff]
With latest libav and audacious-plugins it works, and my patch has been accepted upstream. Closing.