alsa - Add support for media-libs/alsa-lib (Advanced Linux Sound Architecture)
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
ffmpeg - Enable ffmpeg/libav-based audio/video codec support
gles2-only - Use GLES 2.0 or later instead of full OpenGL
gstreamer - Enable audio support via media-libs/gstreamer