I started writing a music visualisation app a while back which worked by using the output of a pulseaudio monitor, so it could visualise the audio from any application. It worked pretty well with the monitor for the speaker output but there seems to be a bug in the audio stack which results in the output from the headphone monitor having incorrect timestamping (at least when read through the GStreamer pulsesrc element) that then causes synchronisation problems for the visualisation elements, which is why I eventually abandoned it.

I'll upload the source when I get home, as it might still be a good starting point for anyone wanting to tackle this project.