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
gstreamer - Add support for media-libs/gstreamer (Streaming media)
gtk - Add support for x11-libs/gtk+ (The GIMP Toolkit)
jack - Add support for the JACK Audio Connection Kit
mplayer - Enable mplayer support for playback or encoding