Soundcard 'monitor' stream doesn't arrive with Spectrum plugin

[Cefn Hoile]

I'm hoping to base a tutorial demo on the first python example linked
below, and I attempted to test its logic by playing music from
Rhythmbox through my (Ubuntu Oneiric) soundcard while visualising the
result of the 'spectrum' FFT as changing numbers streaming on the
console. Sadly this first debugging step was a failure.

http://pzwart3.wdka.hro.nl/wiki/Spectrum.py

The spectrum plugin loaded by this python script only ever reports -60
decibels for every frequency band in the spectrum, regardless how loud
I turn up the music!

I have tried both alsasrc and pulsesrc with various device names
(default, pulse, the introspected alsa monitor device name from pactl)
at the beginning of the listener_desc pipeline (declared in the linked
code). In most of the cases I've tried (barring typos) the pipeline
seems to be established and run perfectly fine (as far as I can tell)
but somehow no sound frequencies show up in the frequency analysis
from the spectrum plugin.

Does this python code work for anyone else? How would I verify that
the pipeline is correct, or narrow down the problem? Any suggestions
what the initial source plugin or entire pipeline should in fact be on
an Ubuntu system to get loudnesses per frequency range streaming out
of the spectrum plugin to the console using this code as a reference?

Thanks for any help.

Cefn
http://cefn.com