How can I recognize YouTube music on Arduino IDE or C?

First of all, the compiler is a friend who changes the source code to something else (usually machine language, byte code, etc.), but it does not work at runtime, such as recognizing YouTube videos.
I think you should study what a compiler is.

There are many ways to make it, but if I were to use the operating system API, I would receive the sound data currently played on the computer and send it to Arduino through serial communication through preprocessing.
(If you are simply going to use the PCM value as the brightness of the LED, you don't need a preprocessing process, do you?)
In Arduino, you can control the port connected to the LED with the received value.

Thank you!