Music from another app or tab? Choose Use microphone, then pick System audio under Source in settings (S). Unlike tab audio, it doesn't show a sharing bar.
Tip: press F for fullscreen, ? for shortcuts. The UI hides itself when you stop moving the mouse.
Using a projector? Put this window on it and open the controls in another window.
Controls on another computer? Download the LAN server and run it with Node.js on this computer: node visualisationator-lan.mjs