aacgain : a port of mp3gain that also works with .m4a files ( ttps://github.com/dgilman/aacgai )
mp3val : MP3 validator ( http://mp3val.sourceforge.net/ )
tageditor : A tag editor with command-line interface supporting MP4/M4A/AAC (iTunes), ID3, Vorbis, Opus, FLAC and Matroska ( https://github.com/Martchus/tageditor )
tagparser : C++ library for reading and writing MP4/M4A/AAC (iTunes), ID3, Vorbis, Opus, FLAC and Matroska tags ( https://github.com/Martchus/tagparser )
Add an ebuild in portage :
The ebuild is now in the portage tree.
You can also use layman : emerge layman then layman -a salfter
For Paludis use this rsync : rsync://gentoo.zugaina.org/salfter-portage
If you have a problem : ycarus(-at-)zugaina.org