In the absence of any instructions being provided, here’s how to install the equalizer plugin referred to here
The goal is to copy the equalizer folder into one of Rhythymbox’s plugin search paths. To find out where your copy of Rhythymbox looks for plugins, run Rhythymbox from the command line like this:
rhythmbox -d 2>&1 | grep "plugin search path"
You should get some output like this:
user@work:/usr/share/rhythmbox/plugins$ rhythmbox -d 2>&1 | grep "plugin search path"
(10:02:33) [0xf93090] [construct_plugins] rb-shell.c:983: plugin search path: /home/user/.local/share/rhythmbox/plugins
(10:02:33) [0xf93090] [construct_plugins] rb-shell.c:991: plugin search path: /usr/lib/rhythmbox/plugins / /usr/share/rhythmbox/plugins
Use the /home location to install just for you, and the /usr/share location (or /usr/local/share location if there is one in your list), to install for all users. For /usr locations, you should in theory put some of the files in /usr/share, and some in /usr/lib, but since no instructions are available as to which files are supposed to go where, I just copy the whole directory into both locations. Assuming you are copying into your home directory:
git clone https://github.com/luqmana/rhythmbox-plugins.git
sudo cp -r equalizer ~/.local/share/rhythmbox/plugins
Now run Rhythymbox again. Go to Edit > Plugins and you should see Equalizer in the list. You should now be able to enable it. If it fails to enable it, and you are running Ubuntu 12.04 64-bit, then you have hit this issue. Do this to resolve it:
sudo apt-get install gir1.2-gconf-2.0