These ebuilds come from http://github.com/sjnewbury/multilib-overlay.
If you have some problems, go to the official site first.
alsa-plugins : ALSA extra plugins ( http://www.alsa-project.org/ )
Add an ebuild in portage :
The ebuild is now in the portage tree.
You can also use layman : emerge layman then layman -a multilib
For Paludis use this rsync : rsync://gentoo.zugaina.org/multilib-portage
If you have a problem : ycarus(-at-)zugaina.org