To make software react to music playing on a computer you must route system audio back in as an input via a platform-specific loopback device

Audio-reactive tools like LedFx analyse an audio input device, but the music you want to react to is playing on an output device — so the output must be routed back to an input. The mechanism is a loopback (a.k.a. monitor / virtual audio cable) and it is platform-specific: on Linux, select ‘pulse’ and capture the ‘Monitor of’ your output in PulseAudio Volume Control; on macOS, install BlackHole and make a Multi-Output Device so sound plays and is captured simultaneously; on Windows, enable ‘Stereo Mix’ for analog output, or — because Stereo Mix cannot capture digital/HDMI output — install a virtual cable such as VB-Audio Voicemeeter and set it as the default playback and recording device. A microphone works too but picks up room noise. This routing step, not the LED hardware, is the most common setup failure.

Examples

Linux: ‘Monitor of Built-in Audio Analog Stereo’; macOS: BlackHole 2ch as input via a Multi-Output Device; Windows HDMI output: VB-Audio Voicemeeter as default playback+recording.

Assessment

Explain why you cannot simply select your speakers as the audio input, and name the correct loopback mechanism for one of Linux, macOS, or Windows.