actually i think that is the difficult part,
as long you use processing and its libraries ( sound / minim / beads … )
to produce “noise” you can know that it is active / or louder as a limit … )
but your question could be
how can processing know if " the computer " plays sound,
for that you would need to be able to read back the system-sound amplitude??
Doing an FFT on system audio currently playing with Minim - Processing Forum like here?