https://askubuntu.com/questions/72679/is-there-any-sound-enhancers-equalizer

Installation Ubuntu 18.04 and later
   sudo apt install pulseaudio-equalizer

Start
   qpaeq
If you encounter an error and are told to make sure you have the pulseaudio 
dbus module loaded follow the instructions in this answer:

https://askubuntu.com/questions/980876/how-do-i-start-pulseaudio-equalizer/982556#982556

then load these two modules using pactl:

   pactl load-module module-equalizer-sink
   pactl load-module module-dbus-protocol

and, to make these changes permanent, edit ~/.config/pulse/default.pa 
(create it if necessary) and add these lines:

   load-module module-equalizer-sink
   load-module module-dbus-protocol

and more proposals