Using Jack, one can route the audio from a Jack enabled audio program to a set of plugins called Meterbridge. Ardour is a Jack enabled audio program that this can be used with.
Jack: http://jackaudio.org/
Ardour: http://ardour.org/
Meterbridge: http://plugin.org.uk/meterbridge/
Unfortunately, it looks like Audacity does not come with any good meters, nor is it Jack capable. It looks like there's an ALSA meter bridge based on the above Jack Meterbridge:
http://laugeo.free.fr/ameter.html
Screenshots to come.