alsa equalizer & loudnorm?

sudo apt install libasound2-plugin-equal
# reboot

in ~/.asoundrc

pcm.!default {
        type plug;
        slave.pcm "plugequal";
}
ctl.equal {
    type equal;
}
pcm.plugequal {
    type equal;
    slave.pcm "plug:dmix";
}

start eq gui with

alsamixer -D equal -g
# -g is for colorless
# or for instant gui
# urxvt -e alsamixer -D equal -g

https://www.alsa-project.org/main/index.php/Asoundrc
http://k.ylo.ph/2016/04/04/loudnorm.html < loudnorm plugin in ffmpeg
http://vlevel.sourceforge.net/using/ < vlevel plugin using LADSPA

The actual alsa loudnorm implementation?

there.