Upstream FFmpeg no longer supports libdacdec as a build-time dependency and the configure flag was removed: https://github.com/FFmpeg/FFmpeg/commit/e259dc86a80398d56d1aabcf6203f80616a4d04d As such, ffmpeg-9999 currently fails building. The fix is trivial, just remove all references to dcadec from the ebuild.
commit 7d6c3ddfd8237b0ff6f62ac9c828cc9057bf7ade Author: Alexis Ballier <aballier@gentoo.org> Date: Mon Mar 28 09:16:06 2016 +0200 media-video/ffmpeg: drop dcadec useflag in live ebuild (dropped upstream)