PulseAudio (tested with version 13.0)
$ make
To use the default PulseAudio device (may not work):
$ ./spectrum_visualizer
To use any other PulseAudio device:
$ ./spectrum_visualizer pulseaudio-device-name
Device names can be found with the pactl command. One of the output devices with the class "monitor" should work.
$ pactl list | grep monitor