A few years ago, I wrote an example on how to port GLSL shader code from Shadertoy.com to Processing. Since then, Shadertoy has changed quite a lot (adding a lot of new uniforms and input methods) so an update was long overdue.
There is still work to be done, but it should be a good starting point for anyone wanting to learn about shaders from the great material available at Shadertoy, and experiment with it in Processing.
I suspect the only way would be using one of the Processing audio libraries (like Sound, Minim, Beads or something else). With such libraries you can load a sound file, play it, get the FFT analysis, and send the FFT values to the shader as a uniform.
Shadertoy plays Soundcloud using their JavaScript API but Soundclound for Java or Processing seems to be no longer maintained since 4 ~ 5 years ago:
But: you could use p5.js, access the Soundcloud API, get the FFT, and forward the data to a shader, everything in the browser (unfortunately without using @sableRaphās library).
How exactly did you copy the Shadertoy and make a .glsl file? I thought I could just make a .txt file and replicate what you had and save it as a .glsl file but that did not work. So how do I save as .glsl files?
You will have to remove text formatting from your file or (better) use a dedicated text editor such as SublimeText, Atom, Brackets, or any of the many text editors designed for code. Copy the glsl code inside the editor, make your changes, and save it directly as a .glsl file. There is no need to go through a .txt step.