alsa - Add support for media-libs/alsa-lib (Advanced Linux Sound Architecture)
cxx - Build support for C++ (bindings, extra libraries, code generation, ...)
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
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
jack - Add support for the JACK Audio Connection Kit