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
flac - Add support for FLAC: Free Lossless Audio Codec
handbook - Enable handbooks generation for packages by KDE