To install Equalizer and other plugins for Rhythmbox in Ubuntu/Linux Mint open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:
After installation go to "Edit" -> "Plugins" -> Enable "Equalizer"
- sudo add-apt-repository ppa:fossfreedom/rhythmbox-plugins
- sudo apt-get update
- sudo apt-get install rhythmbox-plugin-complete
That's it, Enjoy
Source: Ask Ubuntu