I'm currently rewriting this site. Enjoy this Visualizer in the meantime.
On linux choose your audio input device when prompted. Select 'monitor of "your output"' or similar option if available. On windows you have to have "stereo mix" enabled in your audio devices if you want to use your system sound (like music playing on another tab on your browser). On MacOs you need a virtual audio driver like BlackHole or Loopback. Works fine on android. Also you need to give permission to your microphone in your browser
(Effects are affected by the source audio volume level, not by your output device volume.)