So as the title says, I managed to make an in-game music visualizer by animating the scorebar-colour sprite... It just takes a lot of frames/pictures to actually make a visualizer.
This technique requires a lot of frames which means a lot of pictures that Osu! will try to render as possible as it can. It may cause some lag while previewing it in-game or playing with it. Procede at your own risk.
I can only make visualizers for beatmaps that are 1 minute or less because I have trash PC... Please let me know if a fast PC can run Osu! using this technique for 3 to whatever minutes.
This technique can only work on a specific map or else the visualizer will not be synchronized with the song. Make sure you uncheck "Ignore all beatmap skins" in your settings.