Another old old project of mine, original code I think dates back to 2004. This code has been updated with a better FFT and general data processing.
Usage: Just select what device you want to show a spectrum for with the Windows sound device manager (In Windows 10, Settings->System->Sound->Choose your input device)
Originally I used Stereo Mix back in the Windows XP days, so it would just show whatever was playing currently, however these days that option isn't as common.