dir2ogg : Converts mp3, m4a, wma, and wav files to Ogg Vorbis format. ( http://jak-linux.org/projects/dir2ogg/ )
dir2opus : Converts mp3, m4a, wma, and wav files to the Opus format. ( https://github.com/3M3RY/dir2opus )
squishyball : ABX audio comparision tool ( http://svn.xiph.org/trunk/squishyball/ )
Add an ebuild in portage :
The ebuild is now in the portage tree.
You can also use layman : emerge layman then layman -a emery
For Paludis use this rsync : rsync://gentoo.zugaina.org/emery-portage
If you have a problem : ycarus(-at-)zugaina.org