curl - Add support for client-side URL transfer library
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
flac - Add support for FLAC: Free Lossless Audio Codec
jack - Add support for the JACK Audio Connection Kit
libsamplerate - Build with support for converting sample rates using libsamplerate
mad - Add support for mad (high-quality mp3 decoder library and cli frontend)
magic - Add support for file type detection via magic bytes (usually via libmagic from sys-apps/file)
modplug - Add libmodplug support for playing SoundTracker-style music files
musepack - Enable support for the musepack audio codec